Paracetamol: A Focus on Dogs

Corresponding Author: Mario Giorgi Department of Veterinary Sciences, University of Pisa, Pisa, Italy Email: mario.giorgi@unipi.it Abstract: Paracetamol (APAP) is an aniline analgesic, antipyretic and nonnarcotic. It is an essential drug, widely used in human medicine. In veterinary medicine it has an extra label use in many countries. It is used exclusively in some animals, including dogs. It has a mechanism of action similar to that of NSAIDs, as well as other unique characteristics. A variety of studies on APAP in dogs have been published since its introduction into several clinical practices, covering pharmacokinetics, pharmacodynamics, effectiveness and toxicity when inadvertent or accidental overdosing occurs. When taken at therapeutic doses, APAP has been proven to be a powerful and effective analgesic and antipyretic in dogs, as well as having some anti-inflammatory effects. On the other hand, it should be used with caution. This study is a documentation of the therapeutic, toxic and lethal doses of APAP in dogs, as well as the therapeutic effects, clinical application, mostly for the control of post-operative pain and its toxic effects.


Introduction
Paracetamol (acetaminophen or APAP) is one of the most commonly used non-prescription drugs in the world, in human medicine. It is easily accessible and reasonably priced. While APAP is less effective as an anti-inflammatory than Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), it acts better as an analgesic (Belay et al., 1999). It also became the primary analgesic and antipyretic drug during the 1980's after the incident of association of aspirin to Reye's syndrome. It was also safer for children and people with ulcers (Belay et al., 1999).
APAP's essential pharmacological effects are only just recently becoming evident and it is now known to be an inhibitor of Prostaglandin (PG) synthesis in cellular systems under certain conditions. But what about its usage in veterinary medicine?
APAP is not licensed for veterinary usage in United States of America, but is certainly used off label. It is licensed in Europe for oral route in dogs (combined with codeine phosphate) and in pigs (Anonymous, 1999). It is used off-label when administered intravenously and as a single therapeutic agent in non-food producing animals (Serrano-Rodríguez et al., 2019). Large interspecies differences in the metabolic fate of APAP have been observed, making it unsuitable for usage in all animals with a limited usage in veterinary medicine because it is contraindicated in cats, ferrets, hedgehogs, sugar gliders and snakes (Johnston et al., 2002). APAP has a very small therapeutic window in cats and toxicity in these species occurs for doses close to the therapeutic range (10-40 mg/kg). In contrast, toxicity of APAP occurs at higher dosage (>200 mg/kg) in dogs (Savides et al., 1984). It is also generally used in the injectable form in small and large ruminants (Anonymous, 2013).
This review is a snapshot of the current knowledge concerning APAP pharmacology in dogs, focusing on its pharmacokinetics, pharmacodynamics and safety profile.

Nomenclature
The IUPAC name is N-(4-hydroxyphenyl) acetamide. In the United States, Japan, Canada, Venezuela, Colombia and Iran, acetaminophen is the name commonly used, differently the name paracetamol is commonly used in international venues, according to WHO chronicles. It is abbreviated as APAP, for acetyl-para-aminophenol, in some places, such as on prescription bottles of painkillers that contain this drug.

Physicochemical Properties
The compound APAP has a low molecular weight (151.16 g/moL). It is an odorless white crystalline solid with a bitter taste (Lewis, 2007). Since it is such a mild acid (pKa 9.0-9.5), it is effectively unionized at physiological pH levels (Craig, 1990). Its octanol-to-water partition coefficient is 6.2, which is in the range where passive diffusion across cell membranes is possible. Melting point is around 170 °C. It has been found to be very slightly soluble in cold water, but has greater solubility in hot water (14,000 mg/L at 25°C, Yalkowsky et al., 2016). It is freely soluble in alcohol, methanol, ethanol, dimethylformamide, ethylene dichloride, acetone, ethyl acetate, slightly soluble in ether and practically insoluble in petroleum ether, pentane and benzene (O'Neil, 2013).
Chemically, APAP is a phenol and is easily oxidized. APAP synthesis involves three steps starting from phenol. First, phenol is converted to nitro phenol via electrophilic aromatic substitution. Then, the nitro group of the para-substituted nitrophenol is reduced to an amine either by sodium-borohydride (NaBH4) reduction or direct hydrogenation. Finally, the paraaminophenol is converted to APAP via a reaction with acetic anhydride (Ashutosh, 2004). The chemical characteristics of APAP are summarized in Table 1.

Classification and Differentiation from NSAIDs
APAP, is an "aniline analgesic" and it is the only drug of this family still used nowadays. It is the active metabolite of phenacetin, which has fallen out of favor due to its carcinogenic potential in therapeutic doses in humans (IARC, 1987).
Despite their comparable pharmacological function, APAP is not included in the NSAIDs class due to the weak anti-inflammatory activity. When applied in recommended doses, it does not induce, unlike NSAIDs, gastrointestinal side effects. Thus, APAP has not been classified as an NSAID in pharmacological textbooks, despite the fact that it has always been discussed alongside these medications, because of their common functions, mentioned in the Table 2.

Pharmacokinetics
A number of Pharmacokinetic (PK) studies on APAP have been established in dogs. In order to determine the PK profiles, the main analytical technique for APAP concentration detection was the usage of the High Performance Liquid Chromatography (HPLC), coupled to various detectors such as Ultra Violet (UV), Diode Array Detector (DAD) and Mass Spectrometry (MS). The PK were assessed for oral, suppository and intravenous routes of administrations, at different doses. A summary on the analytical methods is described in the Table 3.

Bioavailability
Dogs and most animal species absorb APAP primarily through the small intestine (Gramatté and Richter, 1994;Yamada et al., 1993;Reppas et al., 1998). Its small size, favorable log P and unionized state facilitate diffusion through biological membranes and lead to passive absorption (Swaan et al., 1994). Assuming that the absorption is complete in most species, the first-pass metabolism accounts for the incomplete bioavailability (Rawlins et al., 1977;Perucca and Richens, 1979;Clements et al., 1984). As a result, variations in bioavailability of APAP are most likely due to differences in the degree of first-pass hepatic extraction between organisms and not by absorption. Absorption of readily-soluble drugs is unaffected by gastric and intestinal emptying time (Kelly et al., 2003;Sabnis, 1999). Consequently, the oral bioavailability differences reported in dogs (Neirinckx et al., 2010 44%;Koyanagi et al., 2014 100%) might be assumed to be due to diverse metabolisms in canine breeds (1st pass and glucuronidation) (Bock et al., 2002). A recent study (Sartini et al., 2021), in line with the human findings, affirmed that no statistically significant differences were found between fasted and fed dogs regarding bioavailability, Cmax and Tmax, thus feeding did not significantly affect the APAP absorption process neither its PK.
A study in which APAP was administered rectally showed that it had a much lower bioavailability than orally administered APAP (Sikina et al., 2018). Although it was rapidly absorbed and eliminated, at a dose of 9.5-14 mg/kg, it was unlikely to achieve therapeutic concentrations. Further investigations are recommended, such as improving the formulation, increasing the dose (especially that APAP's toxic dose [200 mg/kg] is far away from the suppository dose given) and adding some absorption enhancers (poloxamer 188 and menthol).
In line with these findings, former studies reported a low rectal bioavailability of human suppository formulations, like tramadol, when administered to dogs (Giorgi et al., 2009).

Plasma Protein Binding and Volume of Distribution
Plasma protein binding of APAP is very low in dogs (Koyanagi et al., 2014). The average protein binding of APAP was between 27% in young dogs and 23% in aged dogs. It was also estimated to be 13% by Duggin and Mudge (1975). As a consequence to this low plasma protein binding, an extensive systemic distribution takes place in dogs, confirmed by the volume of distribution values that ranged from 0.87 to 1.32 L/kg. The large systemic distribution is also a consequence of the small molecular weight of APAP (Martinez, 1998), combined with its unionized state at all physiological pH values. Unlike most conventional NSAIDs, APAP's phenolic structure is more lipophilic than the carboxylic acid structure of NSAIDs (Ali et al., 1996). Very low degree of binding to plasma and serum proteins was also confirmed in humans and pigs (Gazzard et al., 1973;Milligan et al., 1994).

Clearance
Differences in the pharmacokinetic parameters of APAP in different dogs' breeds were found. These differences were assumed to be due to clearance inversely related to body weight (Neirinckx et al., 2010). This is not surprising, given the comparatively larger liver and kidney size, the higher relative amount of hepatic enzymes and number of nephrons in proportion to the weight of kidney tissue in smaller animals, as well as the higher cardiac output and the faster blood flow (Lin, 1995;Toutain and Bousquet-Melou, 2004).
The clearance of APAP in dogs ranged from 0.42 L/h/kg (Sartini et al., 2021) to 1.74 L/h/kg (Neirinckx et al., 2010). The clearance was slower in Labrador retriever dogs compared to that found in Beagles, Greyhounds and Galgo Español dogs (Kukanich, 2010;Neirinckx et al., 2010;Koyanagi et al., 2014;Serrano-Rodríguez et al., 2019). This range may appear wide but pharmacokinetic breed-specific differences are well known in canine species (Fleischer et al., 2008;Martinez et al., 2009;Middleton et al., 2017). These variations must be linked to differences in physical features, body weight and animal size, amount of fat reserves, as well as differences in phase I and II enzyme isoforms involved in drug metabolism (MacNaughton, 2003).
It was anticipated that APAP's clearance in dogs is not influenced by changes in urinary pH within the achievable physiological range since APAP is a weak acid with a pKa of 9.5 (Duggin and Mudge, 1975). The clearance of APAP depends on urine flow rate but not pH, which was similar to results in humans (Prescott, 1980).

Metabolism, Metabolites and Excretion
APAP is mainly metabolized in the liver by phase I and II enzymes. After 24 h, most of the drug is recoverable in the urine as conjugates (Savides et al., 1984). Oxidation, reduction and hydrolysis are all possible phase I reactions for APAP in dogs, however, a small proportion only compared to phase II. For the phase II enzymes, in canine species, as in humans, glucuronidation accounts for the majority of the metabolism of APAP (76%), with a lesser contribution of sulfation and some other pathways (Patel et al., 1992;Prescott, 1983;Savides et al., 1984). Glucuronidation and sulfation yield final products are inactive, nontoxic, hydrophilic and are excreted by the kidneys. However, the small percentage of APAP that is oxidized by Cytochrome P450 (CYP) enzyme transforms to a reactive toxic metabolite N-acetyl-p-benzoquinoneimine (NAPQI) (Davis et al., 1976). At therapeutic doses of APAP, NAPQI binds to Glutathione (GSH) which is a potent tripeptide antioxidant present in all tissues and is then excreted in the urine with the other metabolites, as cysteine and mercapturic acid. The metabolism of APAP in the liver is shown in Fig. 1. Savides et al. (1984) assessed the presence of the metabolites in dogs' urine, at 100 mg/kg APAP administration: APAP-glucuronide (75%), APAP-sulfate (17%), APAP-cysteine (5%) and unchanged APAP (2%). APAP-mercapturic acid accounted for 1%, only after giving a dose of 500 mg/kg. The production of cysteine and mercapturic acid conjugates of APAP is of major toxicological significance (Mitchell et al., 1973;1974;1977).
Concerning the excretion, only a very small amount of APAP is bound to plasma proteins and therefore the major part undergoes glomerular filtration. It is reabsorbed in the renal tubules by simple diffusion. The excretory mechanisms for the conjugates are different from those of the parent APAP compound and the excretory pattern of sulphate and glucuronide conjugates are somewhat different from each other (Duggin and Mudge, 1975). For both, clearance is not affected by urine pH or the rate of urine flow, but is strongly influenced by the concentration of the conjugate in the plasma. Clearance, corrected for plasma binding, shows net tubular secretion at low plasma levels and net reabsorption at high levels. Thus, each conjugate undergoes bidirectional tubular transport.
The sulfate and the glucuronide, both undergo glomerular filtration, being weakly protein bound. At low concentrations in plasma, both compounds are secreted by an active transport process. At higher concentrations, both compounds are reabsorbed. For the reabsorption, APAP itself undergoes reabsorption throughout the nephron while the conjugates are transported in the proximal tubule. The mechanism is explained in details in Duggin and Mudge (1975).
A summary on the studies description, main pharmacokinetic parameters of APAP and safety profiles found in the various literature on dogs, is shown in Table 4 and 5.      agents. It is a potent anti-oxidant, also reduces Evidence of anti-arrhythmic effects the activity of myeloperoxidase (Brennan et al., and heart stabilization too 2002), which in turn significantly reduces the oxidation of low-density lipoproteins (LDLs) in macrophages (Podrez et al., 2000;Golfetti et al., 2003).

Pharmacodynamics
The mechanism of action of APAP, which is established mainly in mice, rats and humans, is not fully understood in dogs yet. Thus this review will briefly discuss the findings on APAP's Pharmacodynamics (PD), followed by evidence on the therapeutic effects found in dogs.
APAP is not directly a PGs synthesis inhibitor. APAP inhibits PG activity by acting as a substrate of the peroxidase cycles of COX-1 and COX-2 but, the main impact is frequently on COX-2 (Boutaud et al., 2002;Graham and Scott, 2005;Aronoff et al., 2006;Graham et al., 2013). When concentrations of arachidonic acid are low, the COX-2 pathway is activated in preference to the COX-1 pathway (Graham and Scott, 2005). APAP can inhibit COX, both centrally and peripherally, when ambient concentrations of peroxides are low. However, under pro-inflammatory conditions, when peroxide concentrations are high, APAP is ineffective peripherally and is only active in the brain, where baseline peroxide concentrations are very low. The inhibition of cerebral COX is responsible for the antipyretic effects of APAP (Ouellet and Percival, 2001).
In dogs, a described third isoform, COX-3, has been identified in the cerebral cortex, with minimal amounts found peripherally. This new enzyme was discovered to be inhibited by APAP (Jóźwiak-Bebenista and Nowak, 2014;Chandrasekharan et al., 2002). However, its activity and physiological effects in dogs, rats and humans have been the source of some debate and speculation (Kis et al., 2005;Lucas et al., 2005).
Regarding the therapeutic effects in dogs, APAP is safe when prescribed at a therapeutic dose and for a limited period of time (Serrano-Rodríguez et al., 2019). In all the studies, it has been noticed that no visible side effects are seen with APAP doses below 100 mg/kg. Many studies have been established, however, further investigations are needed. For instance, the plasma concentration of APAP that can provide analgesia in dogs is unknown. A study published in 2006 reported that a plasma concentration close to 4 μg/mL was sufficient to provide analgesia in humans (Pickering et al., 2006). Despite that the PK/PD relationship for most of the analgesic or an anti-inflammatory drugs obeys to some indirect effects (Sharma and Jusko, 1998), oral acetaminophen in humans suggests to have a minimal hysteresis (nearly a direct effect) (Pickering et al., 2006). Then, if assumed that dogs and humans have the same minimal effective concentration, plasma concentrations of APAP above 4 μg/mL might provide antinociceptive effect for a few hours (Giorgi et al., 2012;Giorgi et al., 2016).
Further evidencing its potential for post-surgical use, the administration of the recommended dose of APAP in dogs (20 mg/kg every 8 h) (Sartini et al., 2021) can be used instead of NSAIDs, especially if these are contraindicated (Berry, 2015). To note that recently, a recommended drug combination suggested for analgesia in dogs, is an oral opioid formulation plus APAP (Plumb, 2015;Muir, 2015). Opioids that have been combined with APAP for this purpose include codeine, oxycodone and hydrocodone (approved for usage in Europe) (Egger et al., 2014;Benitez et al., 2015;Kukanich, 2010).
APAP is also included in opioid-free anaesthesia protocols, which are often combined with other anesthetic/analgesic drugs, including medetomidine, ketamine, lidocaine, bupivacaine, carprofen and meloxicam in dogs (White et al., 2017).
The documented therapeutic effects of APAP in dogs are summarized in Table 6.

Toxicology and Pathology
The clinical signs of APAP toxicity are generally seen with doses above 150 mg/kg (St. Omer and Mohamed, 1984). APAP is one of the most common household medications and it is not surprising that APAP toxicity, as an unintentional or accidental overdose in dogs, is frequently reported (Caloni et al., 2014).
The APAP is often poorly used in veterinary medicine because of the wrong belief that it possesses a narrow therapeutic index and several potential increases in toxicity when used in combination with other drugs or natural compounds. Concerning the drug-drug interaction and the resulting toxicity, it was affirmed that maximal enzymatic induction with ethanol in humans is not capable of increasing APAP toxicity when administered within the therapeutic range (Thummel et al., 2000;Rumack, 2004). Phenytoin was also thought to enhance APAP toxicity (Manyike et al., 2000;Brackett and Bloch, 2000). As a CYP3A4 inducer, it does not increase APAP toxicity. Indeed CYP3A4 accounts for only a small portion of APAP metabolism. CYP2E1 is the principal metabolic enzyme for APAP metabolism to NAPQI. Another wrong theory is that barbiturates (i.e., phenobarbital), acting as a pleiotropic inducer of phase I and phase II reactions, can induce all the metabolic enzymes and consequently the CYP2E1. If theoretically this hypothesis has some basis, it has been experimentally assessed that phenobarbital has no effect on any of the processes of APAP-toxic metabolites (Rumack, 2002;.

General Toxicity
At toxic doses (> 150 mg/kg), sulfate and glucuronosyl transferases become saturated and NAPQI production increases. If GSH is depleted to < 20% of its usual concentration, NAPQI binds covalently to cysteine groups on hepatocellular proteins via cysteine residues, disrupting cellular integrity and yielding hepatocyte necrosis (Pumford et al., 1990). Most of the covalent binding occurs centrolobularly, being preferentially localized in the endoplasmic reticulum and in the enzymes of the cytoplasm. This injury likely takes place very rapidly once GSH depletion is accomplished, leading to the extraordinary levels of aminotransferases and other cellular enzymes, but also a very rapid decline upon cessation of liver injury. Likewise, a free radical formed through the Mixed Function Oxidase (MFO) system causes oxidative damage to cellular molecules (Pereira et al., 1992).

Nephrotoxicity
Renal damage is a secondary effect described following APAP administration (Salem et al., 2010). Nephrotoxicity is caused by a deacetylation of APAP in the kidney to form p-aminophenol (PAP), a minor metabolite, however a potent nephrotoxin (Carpenter and Mudge, 1981;Crowe et al., 1979). This compound may be oxidized to p-benzoquinoneimine, which is very unstable and has a cytotoxicity comparable to NAPQI. Although produced by different metabolic routes, PAP and NAPQI can be produced in the kidney (Bessems and Vermeulen, 2001). These two compounds produced severe congestion of the cortex and medulla, proteinaceous tubular casts and nephrosis after 500 mg/kg APAP administration (Savides et al., 1984).
A 200 mg/kg dose produced an increased echodensity in kidney parenchyma that matched with renal damage in dogs. In Salem et al. (2010), renal smears upon cytology showed moderate to severe degree of vacuolation and degeneration of cells and tubular cells degenerated into dark gray amorphous debris representing the necrotic material. This nephrotoxicity is most likely attributed to a depletion of GSH in the renal parenchyma (Loh and Ponampalam, 2006;Kurtovic and Riordan, 2003). On histology, congestion with vasculitis, thickened renal capsule (perinephritis), vacuolation of the glomerular and tubular epithelium, necrosis and perivascular fibrosis were observed. Increased concentrations of Blood Urea Nitrogen (BUN) and serum creatinine reflect this renal damage and were consistent in all reports (Savides et al., 1984;Salem et al., 2010;MacNaughton, 2003;Ortega et al., 1985;Savides and Oehme, 1983;Schlesinger, 1995).

Hematotoxicity
APAP hematotoxicity in dogs is mainly attributed to PAP (Mc Conkey et al., 2009;Allen, 2003;Taylor and Dhupa, 2003). The toxic metabolites bind to iron and cellular material, resulting in methemoglobinemia, membrane oxidative injury and Heinz bodies formation (Rianprakaisang et al., 2019). The deficiency of arylamine N-acetyltransferase (NAT) activity (polymorphic cytosolic conjugating enzymes) in dogs (and cats) contributes to this species-dependent methemoglobinemia (Mc Conkey et al., 2009). This blood toxicity does not occur in all cases. It is claimed to be a chronic consequence after a long term administration, however, in many reports it seems to be an acute symptom, with or without hepatotoxicity (Schlesinger, 1995;MacNaughton, 2003).
In all of the references mentioned above for dogs, the hematology repercussions were similar. A mild to severe regenerative anemia, accompanied by a mild to severe stress leukogram were noted. In all reports, significant neutrophilia was consistent. In some cases, there were fragmented red blood cells, poikilocytosis, mild agglutination, spherocytes, acanthocytes, anisocytosis and polychromasia (MacNaughton, 2003;Schlesinger, 1995;Salem et al., 2010;Harvey et al., 1986).

Hepatotoxicity
The clinical severity of hepatotoxicity is proportional to the dose and ranges from mild to severe acute hepatitis. Liver lesions were similar in most studies (Ortega et al., 1985;Gazzard et al., 1975;Salem et al., 2010) and analogous to morphological changes described by other authors in man and in several animal species (McGregor et al., 2003;Sheen, 2002;Dixon et al., 1975;Mitchell, 1977).
Dogs receiving 250 mg/kg showed acute hepatitis and focal necrosis in the centrilobular region with inflammatory infiltrates. Some livers, in addition, showed granulomatous aggregates in acinus and portal space consisting of epithelioid cells with peripheral lymphocyte infiltration (Ortega et al., 1985).
Dogs receiving 500 mg/kg (lethal dose), all died after 76 h and showed massive hepatic necrosis extended from terminal hepatic venules to portal spaces, hyperemic sinusoids and hypertrophic sinusoidal cells. Subcellular changes also took place with formation of lamellar structures on the nucleus and mitochondria (Dixon et al., 1975).
Similar liver lesions were also found, with congestion, extensive necrosis, fatty vacuoles at an APAP dose of 3000 mg/kg (Gazzard et al., 1975). All dogs died in approximately 8 h. Raised levels of arterial ammonia, reduced arterial partial pressure of oxygen, methemoglobinemia and markedly increased Aspartate Aminotransferase (AST) levels occurred for those who survived more than 24 h.
The ingested dose or, more precisely, plasma concentrations of APAP, can predict the incidence and severity of hepatotoxicity. Only when the time of acute ingestion of APAP is known, the Rumack-Matthew nomogram is used to estimate the probability of hepatotoxicity. Plasma concentrations higher than 150 μg/mL suggest possible hepatotoxicity in humans (Rumack and Matthew, 1975). Figure 2 represents the nomogram consisting of a semi-logarithmic curve of plasma APAP levels versus time. concentration is plotted on the graph using the time from overdose to blood draw. If above the risk line, 150 g/mL, hepatotoxicity is possible and the patient receives acetylcysteine treatment at a dose of 300 mg/kg body weight. Toxicity is very probable above 200 g/mL. If the plasma concentration is below 150 g/mL, hepatotoxicity is unlikely and no need for treatment This approach is established in human medicine but with some adjustments it might also fit in canines. Additional tests are recommended if poisoning is confirmed or highly suspected, or if the time of consumption is uncertain. If severe intoxication is suspected, liver enzymes tests and prothrombin time should be conducted. The AST and ALT levels appear to be proportionally related to the stage of poisoning. Bilirubin also increases if the intoxication is severe (O'Malley and O'Malley, 2020).

Antidotes against APAP Toxicity
N-acetylcysteine (NAC), the precursor of GSH, is a specific antidote against APAP toxicosis in dogs and cats (St. Omer and McKnight, 1980;St. Omer and Mohammad, 1984) and liver necrosis in man (Prescott and Wright, 1973). It is currently the only FDA approved antidote for APAP overdose in humans (Khayyat et al., 2016). NAC restores GSH levels which acts directly on NAPQI to form an acetyl-cysteine conjugate which is excreted in bile. Additionally, NAC supplies mitochondrial energy substrates in the Krebs cycle and restores hepatic ATP levels by providing excess amino-acid and uses it as energy substrates (Saito et al., 2010;Lauterburg et al., 1983).
The minimum recommended clinical dosing schedule of NAC for the treatment of APAP toxicosis in dogs is 140 mg/kg orally, repeated every 4 hours for three treatments (St. Omer and McKnight, 1980). It has been reported that NAC alters the pharmacokinetics of APAP (St. Omer and Mohammad, 1984). It decreased the elimination terminal half-life of APAP by 40% and increased its clearance by 60%. Similar results were obtained in rats (Galinsky and Levy, 1979).
Moreover, it has been demonstrated that cimetidine, an inhibitor of some cytochrome oxidase enzymes, decreases the production of NAPQI by blocking CYP 450 (Ruepp et al., 2002). This would be of benefit to species that develop centrolobular necrosis due to NAPQI, like dogs (Sajedianfard et al., 2009;Rudd et al., 1981;Mitchell et al., 1984).

Conclusion
APAP, when used in the therapeutic levels, has shown to be a potent and effective analgesic and antipyretic in dogs, with some anti-inflammatory activity. When used in doses below 100 mg/kg, no side effects occur and at recommended therapeutic levels, generally between 10 and 20 mg/kg, is effective for postoperative pain control. It can also be used instead of NSAIDs when these are contraindicated, in combination with opiods and in opioid-free anesthesia surgery protocols. APAP also showed cardioprotective and anti-arrhythmic effects in dogs, nevertheless more details are needed. APAP level (g/ml) APAP, however, must be used with caution. Doses above 150 mg/kg are toxic and the repercussions are severe, with hepatotoxicity, hematotoxicity and nephrotoxicity. Doses above 250 mg/kg can be lethal. Antidotes of APAP such as NAC and cimetidine are shown to effectively reverse, partially, the toxicity.

Acknowledgement
No third-party funding or support was received in connection with this study or the writing or publication of the review. Authors acknowledge the English editing to Dr Ritta Hanna (Bsc)

Author's Contributions
Charbel Fadel: Developed the literature search and wrote the draft version of the review. Reviewed and approved the final version of the paper.
Irene Sartini: Contributed in the literature search, planned tables and plots. Verified the consiastency of the information. Reviewed and approved the final version of the paper.
Mario Giorgi: Conceived of the presented idea and supervised the project. Provided critical feedback and helped shape manuscript Reviewed and approved the final version of the paper.