Adult Population
A total of 1,020 adult patients have received acetaminophen in clinical trials, including 37.3% (n=380) who received 5 or more doses, and 17% (n=173) who received more than 10 doses. Most patients were treated with acetaminophen 1,000 mg every 6 hours. A total of 13.1% (n=134) received acetaminophen 650 mg every 4 hours.
All adverse reactions that occurred in adult patients treated with either acetaminophen or placebo in repeated dose, placebo-controlled clinical trials at an incidence ≥ 3% and at a greater frequency than placebo are listed in Table 3. The most common adverse events in adult patients treated with acetaminophen (incidence ≥ 5% and greater than placebo) were nausea, vomiting, headache, and insomnia.
Table 3. Treatment-Emergent Adverse Reactions Occurring in ≥ 3% of Acetaminophen- treated Adult Patients and at a Greater Frequency than Placebo in Placebo-Controlled, Repeated Dose Studies
|
System Organ Class - Preferred Term | Acetaminophen (N=402) n (%) | Placebo (N=379) n (%) |
Gastrointestinal Disorders Nausea Vomiting
| 138 (34) 62 (15)
| 119 (31) 42 (11)
|
General Disorders and Administration Site Conditions Pyrexia*
| 22 (5)
| 52 (14)
|
Nervous System Disorders Headache
| 39 (10)
| 33 (9)
|
Psychiatric Disorders Insomnia
| 30 (7)
| 21 (5)
|
Other Adverse Reactions Observed During Clinical Studies of Acetaminophen in Adults
The following additional treatment-emergent adverse reactions were reported by adult subjects treated with acetaminophen in all clinical trials (n=1,020) that occurred with an incidence of at least 1% and at a frequency greater than placebo (n=525).
Blood and lymphatic system disorders: anemia
General disorders and administration site conditions: fatigue, infusion site pain, edema peripheral
Investigations: aspartate aminotransferase increased, breath sounds abnormal
Metabolism and nutrition disorders: hypokalemia
Musculoskeletal and connective tissue disorders: muscle spasms, trismus
Psychiatric disorders: anxiety
Respiratory, thoracic and mediastinal disorders: dyspnea
Vascular disorders: hypertension, hypotension
Pediatric Population
A total of 355 pediatric patients (47 neonates, 64 infants, 171 children, and 73 adolescents) have received acetaminophen in active-controlled (n=250) and open-label clinical trials (n=225), including 59.7% (n=212) who received 5 or more doses and 43.1% (n=153) who received more than 10 doses. Pediatric patients received acetaminophen doses up to 15 mg/kg on an every 4 hours, every 6 hours, or every 8 hours schedule. The maximum exposure was 7.7, 6.4, 6.8, and 7.1 days in neonates, infants, children, and adolescents, respectively.
The most common adverse events (incidence ≥ 5%) in pediatric patients treated with acetaminophen were nausea, vomiting, constipation, pruritus, agitation, and atelectasis.
Other Adverse Reactions Observed During Clinical Studies of Acetaminophen in Pediatrics The following additional treatment-emergent adverse reactions were reported by pediatric subjects treated with acetaminophen (n=355) that occurred with an incidence of at least 1%.
Blood and lymphatic system disorders: anemia
Cardiac disorders: tachycardia
Gastrointestinal disorders: abdominal pain, diarrhea
General disorders and administration site conditions: pyrexia, injection site pain, edema peripheral
Metabolism and nutrition disorders: hypokalemia, hypomagnesemia, hypoalbuminemia, hypophosphatemia, hypervolemia
Musculoskeletal and connective tissue disorders: muscle spasm, pain in extremity
Nervous system disorders: headache
Psychiatric disorders: insomnia
Renal and urinary disorders: oliguria
Respiratory, thoracic and mediastinal disorders: hypoxia, pleural effusion, pulmonary edema, stridor, wheezing
Skin and subcutaneous tissue disorders: periorbital edema, rash
Vascular disorders: hypotension, hypertension
Risk Summary
Prolonged experience with acetaminophen in pregnant women over several decades, based on published observational epidemiological studies and case reports, did not identify a drug associated risk of major birth defects, miscarriage, or adverse maternal or fetal outcomes (see Data). Animal reproduction studies have not been conducted with IV acetaminophen. Reproductive and developmental studies in rats and mice from the published literature identified adverse events at clinically relevant doses with acetaminophen. Treatment of pregnant rats with doses of acetaminophen approximately equal to the maximum human daily dose (MHDD) showed evidence of fetotoxicity and increases in bone variations in the fetuses. In another study, necrosis was observed in the liver and kidney of both pregnant rats and fetuses at doses approximately equal to the MHDD. In mice and rats treated with acetaminophen at doses within the clinical dosing range, cumulative adverse effects on reproductive capacity were reported. In mice, a reduction in number of litters of the parental mating pair was observed as well as retarded growth, abnormal sperm in their offspring and reduced birth weight in the next generation. In rats, female fertility was decreased following in utero exposure to acetaminophen (see Data).
The estimated background risk of major birth defects and miscarriages for the indicated population is unknown. All pregnancies have a background risk of birth defect, loss, or other adverse outcomes. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2-4% and 15-20%, respectively.
Data
Human Data
The results from a large population-based prospective cohort, including data from 26,424 women with live born singletons who were exposed to oral acetaminophen during the first trimester, indicate no increased risk for congenital malformations, compared to a control group of unexposed children. The rate of congenital malformations (4.3%) was similar to the rate in the general population. A population-based, case-control study from the National Birth Defects Prevention Study showed that 11,610 children with prenatal exposure to acetaminophen during the first trimester had no increased risk of major birth defects compared to 4,500 children in the control group. Other epidemiological data showed similar results. However, these studies cannot definitively establish the absence of any risk because of methodological limitations, including recall bias.
Animal Data
Studies in pregnant rats that received oral acetaminophen during organogenesis at doses up to 0.85 times the maximum human daily dose (MHDD = 4 grams/day, based on a body surface area comparison) showed evidence of fetotoxicity (reduced fetal weight and length) and a dose-related increase in bone variations (reduced ossification and rudimentary rib changes). Offspring had no evidence of external, visceral, or skeletal malformations. When pregnant rats received oral acetaminophen throughout gestation at doses of 1.2 times the MHDD (based on a body surface area comparison), areas of necrosis occurred in both the liver and kidney of pregnant rats and fetuses. These effects did not occur in animals that received oral acetaminophen at doses 0.3 times the MHDD, based on a body surface area comparison.
In a continuous breeding study, pregnant mice received 0.25, 0.5, or 1% acetaminophen via the diet (357, 715, or 1,430 mg/kg/day). These doses are approximately 0.43, 0.87, and 1.7 times the MHDD, respectively, based on a body surface area comparison. A dose related reduction in body weights of fourth and fifth litter offspring of the treated mating pair occurred during lactation and post-weaning at all doses. Animals in the high dose group had a reduced number of litters per mating pair, male offspring with an increased percentage of abnormal sperm, and reduced birth weights in the next generation pups.
Risk Summary
There is no information regarding the presence of acetaminophen in human milk, the effects on the breastfed infant, or the effects on milk production. However, limited published studies report that acetaminophen passes rapidly into human milk with similar levels in the milk and plasma. Average and maximum neonatal doses of 1% and 2%, respectively, of the weight-adjusted maternal dose are reported after a single oral administration of 1 gram APAP. There is one well-documented report of a rash in a breast-fed infant that resolved when the mother stopped acetaminophen use and recurred when she resumed acetaminophen use. The developmental and health benefits of breastfeeding should be considered along with the mother's clinical need for acetaminophen injection and any potential adverse effects on the breastfed infant from acetaminophen injection or from the underlying maternal condition.
Treatment of Acute Pain
The safety and effectiveness of acetaminophen for the treatment of acute pain in pediatric patients ages 2 years and older is supported by evidence from adequate and well- controlled studies of acetaminophen in adults and safety and pharmacokinetic data from adult and 355 pediatric patients across all age groups [see Dosage and Administration (2.3) and Pharmacokinetics (12.3)].
The effectiveness of acetaminophen for the treatment of acute pain in pediatric patients younger than 2 years of age has not been established.
Treatment of Fever
The safety and effectiveness of acetaminophen injection for the treatment of fever in pediatric patients is supported by adequate and well-controlled studies of acetaminophen injection in adults and clinical studies in 244 pediatric patients 2 years and older.
Signs and Symptoms
In acute acetaminophen overdosage, dose-dependent, potentially fatal hepatic necrosis is the most serious adverse effect. Renal tubular necrosis, hypoglycemic coma, and thrombocytopenia may also occur. Plasma acetaminophen levels > 300 mcg/mL at 4 hours after oral ingestion were associated with hepatic damage in 90% of patients; minimal hepatic damage is anticipated if plasma levels at 4 hours are < 150 mcg/mL or < 37.5 mcg/mL at 12 hours after ingestion. Early symptoms following a potentially hepatotoxic overdose may include: nausea, vomiting, diaphoresis, and general malaise. Clinical and laboratory evidence of hepatic toxicity may not be apparent until 48 to 72 hours post-ingestion.
Treatment
If an acetaminophen overdose is suspected, obtain a serum acetaminophen assay as soon as possible, but no sooner than 4 hours following oral ingestion. Obtain liver function studies initially and repeat at 24-hour intervals. Administer the antidote N-acetylcysteine (NAC) as early as possible. As a guide to treatment of acute ingestion, the acetaminophen level can be plotted against time since oral ingestion on a nomogram (Rumack-Matthew). The lower toxic line on the nomogram is equivalent to 150 mcg/mL at 4 hours and 37.5 mcg/mL at 12 hours. If serum level is above the lower line, administer the entire course of NAC treatment. Withhold NAC therapy if the acetaminophen level is below the lower line.
For additional information, call a poison control center at 1-800-222-1222.
Distribution
The pharmacokinetics of acetaminophen have been studied in patients and healthy subjects up to 60 years old. The pharmacokinetic profile of acetaminophen has been demonstrated to be dose proportional in adults following administration of single doses of 500, 650, and 1,000 mg.
The maximum concentration (Cmax) occurs at the end of the 15-minute intravenous infusion of acetaminophen. Compared to the same dose of oral acetaminophen, the Cmax following administration of acetaminophen is up to 70% higher, while overall exposure (area under the concentration time curve [AUC]) is very similar.
Pharmacokinetic parameters of acetaminophen (AUC, Cmax, terminal elimination half-life [T1/2], systemic clearance [CL], and volume of distribution at steady state [Vss]) following administration of a single intravenous dose of 15 mg/kg in children and adolescents and 1,000 mg in adults are summarized in Table 4.
Table 4. Acetaminophen Pharmacokinetic Parameters
Subpopulations | Mean (SD) |
AUC0-6h (μg x h/mL) | Cmax (μg/mL) | T1/2 (h) | CL (L/h/kg) | Vss (L/kg) |
| Neonates | 62 (11)
| 25 (4)
| 7 (2.7)
| 0.12 (0.04)
| 1.1 (0.2)
|
| Infants | 57 (54)
| 29 (24)
| 4.2 (2.9)
| 0.29 (0.15)
| 1.1 (0.3)
|
| Children | 38 (8)
| 29 (7)
| 3 (1.5)
| 0.34 (0.10)
| 1.2 (0.3)
|
| Adolescents | 41 (7)
| 31 (9)
| 2.9 (0.7)
| 0.29 (0.08)
| 1.1 (0.3)
|
| Adults | 43 (11)
| 28 (21)
| 2.4 (0.6)
| 0.27 (0.08)
| 0.8 (0.2)
|
The pharmacokinetic exposure of acetaminophen observed in children and adolescents is similar to adults, but higher in neonates and infants. Dosing simulations from pharmacokinetic data in infants and neonates suggest that dose reductions of 33% in infants 1 month to < 2 years of age, and 50% in neonates up to 28 days, with a minimum dosing interval of 6 hours, will produce a pharmacokinetic exposure similar to that observed in children age 2 years and older.
At therapeutic levels, binding of acetaminophen to plasma proteins is low (ranging from 10% to 25%). Acetaminophen appears to be widely distributed throughout most body tissues except fat.
Metabolism and Excretion
Acetaminophen is primarily metabolized in the liver by first-order kinetics and involves three principal separate pathways: Conjugation with glucuronide, conjugation with sulfate, and oxidation via the cytochrome P450 enzyme pathway, primarily CYP2E1, to form a reactive intermediate metabolite (N-acetyl-p-benzoquinone imine or NAPQI). With therapeutic doses, NAPQI undergoes rapid conjugation with glutathione and is then further metabolized to form cysteine and mercapturic acid conjugates.
Acetaminophen metabolites are mainly excreted in the urine. Less than 5% is excreted in the urine as unconjugated (free) acetaminophen and more than 90% of the administered dose is excreted within 24 hours.
Carcinogenesis
Long-term studies in mice and rats have been completed by the National Toxicology Program to evaluate the carcinogenic potential of acetaminophen. In 2-year feeding studies, F344/N rats and B6C3F1 mice were fed a diet containing acetaminophen up to 6,000 ppm. Female rats demonstrated equivocal evidence of carcinogenic activity based on increased incidences of mononuclear cell leukemia at 0.8 times the maximum human daily dose (MHDD) of 4 grams/day, based on a body surface area comparison. In contrast, there was no evidence of carcinogenic activity in male rats (0.7 times) or mice (1.2-1.4 times the MHDD, based on a body surface area comparison).
Mutagenesis
Acetaminophen was not mutagenic in the bacterial reverse mutation assay (Ames test). In contrast, acetaminophen tested positive in the in vitro mouse lymphoma assay and the in vitro chromosomal aberration assay using human lymphocytes. In the published literature, acetaminophen has been reported to be clastogenic when administered a dose of 1,500 mg/kg/day to the rat model (3.6 times the MHDD, based on a body surface area comparison). In contrast, no clastogenicity was noted at a dose of 750 mg/kg/day (1.8 times the MHDD, based on a body surface area comparison), suggesting a threshold effect.
Impairment of Fertility
In studies conducted by the National Toxicology Program, fertility assessments have been completed in Swiss mice via a continuous breeding study. There were no effects on fertility parameters in mice consuming up to 1.7 times the MHDD of acetaminophen, based on a body surface area comparison. Although there was no effect on sperm motility or sperm density in the epididymis, there was a significant increase in the percentage of abnormal sperm in mice consuming 1.7 times the MHDD (based on a body surface area comparison) and there was a reduction in the number of mating pairs producing a fifth litter at this dose, suggesting the potential for cumulative toxicity with chronic administration of acetaminophen near the upper limit of daily dosing.
Published studies in rodents report that oral acetaminophen treatment of male animals at doses that are 1.2 times the MHDD and greater (based on a body surface area comparison) result in decreased testicular weights, reduced spermatogenesis, reduced fertility, and reduced implantation sites in females given the same doses. These effects appear to increase with the duration of treatment.
In a published mouse study, oral administration of 50 mg/kg acetaminophen to pregnant mice from Gestation Day 7 to delivery (0.06 times the MHDD, based on a body surface area comparison) reduced the number of primordial follicles in female offspring and reduced the percentage of full term pregnancies and number of pups born to these females exposed to acetaminophen in utero.
In a published study, oral administration of 350 mg/kg acetaminophen to pregnant rats (0.85 times the MHDD, based on a body surface area comparison) from Gestation Day 13 to 21 (dams) reduced the number of germ cells in the fetal ovary, decreased ovary weight, and reduced the number of pups per litter in F1 females as well as reduced ovary weights in F2 females.
Pain Study 1 evaluated the analgesic efficacy of repeated doses of acetaminophen 1,000 mg vs. placebo every 6 hours for 24 hours in 101 patients with moderate to severe pain following total hip or knee replacement. Acetaminophen was statistically superior to placebo for reduction in pain intensity over 24 hours. There was an attendant decrease in opioid consumption, the clinical benefit of which was not demonstrated.
Pain Study 2 evaluated the analgesic efficacy of repeated doses of acetaminophen 1,000 mg every 6 hours or 650 mg every 4 hours for 24 hours versus placebo in the treatment of 244 patients with moderate to severe postoperative pain after abdominal laparoscopic surgery.
Patients receiving acetaminophen experienced a statistically significant greater reduction in pain intensity over 24 hours compared to placebo.