Acetaminophen: 7 things you should know

Medically reviewed by Carmen Pope, BPharm. Last updated on Nov 28, 2024.

1. How it works

• Acetaminophen is used to relieve pain.
• Experts aren't sure exactly how acetaminophen works but suspect it blocks a specific type of cyclo-oxygenase (COX) enzyme, located mainly in the brain.
• Acetaminophen belongs to the class of medicines called analgesics (pain relievers); it is specifically a non-narcotic analgesic. It may also be called an antipyretic because it will bring down a fever.

2. Upsides

• Acetaminophen is used to treat pain and helps temporarily relieve minor aches, pains, and headaches caused by conditions such as arthritis, back pain, colds, menstruation pain, and toothache.
• Lowers a fever but does not control inflammation.
• First choice for mild-to-moderate pain due to its efficacy, minimal toxicity, and low cost.
• At low dosages, it lacks the gastrointestinal side effects associated with NSAID pain relievers (does not cause ulcerations, bleeding, or perforations).
• May be combined with strong opioids for the treatment of severe pain.
• Available in a range of formulations including liquid, oral tablets, oral disintegrating tablets, chewable tablets, controlled-release tablets, dispersible tablets, injectable, and rectal preparations.
• Available as a fixed combination with several other agents, such as aspirin, caffeine, chlorpheniramine, or dextromethorphan. Always consider the precautions, cautions, and contraindications associated with the concomitant agent.
• Considered acceptable to use intermittently during pregnancy and breastfeeding. IV acetaminophen has not been studied in pregnant women; use only after assessment of risks versus benefits.
• Generic acetaminophen is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

• Side effects of acetaminophen may include itchiness, constipation, nausea, vomiting, headache, insomnia, or agitation, but these are not common. May cause gastrointestinal side effects at high dosages.
• The potential for liver damage exists, even at recommended dosages. The risk is increased with higher dosages, a shorter interval between doses, in people who drink three or more alcoholic drinks per day, when taken with other medications that also contain acetaminophen, and in patients with pre-existing liver disease.
• Considered less effective for pain relief than NSAIDs but is associated with a lower risk of side effects.
• May not be suitable for some people including people who regularly consume three or more glasses of alcohol per day and those with liver disease.
• IV acetaminophen has been associated with dosage and administration errors that have led to accidental overdosage and death. Be careful when prescribing and preparing the injectable preparation. Ensure that the dose in mg and the volume in mL are not confused. The dosage for people < 50kg is based on body weight, the infusion pump is set up properly, and the total dose of acetaminophen from all sources does not exceed the maximum recommended dose.
• May interact with some other medications including warfarin.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

• Acetaminophen is an effective mild pain reliever with a low risk of side effects. It carries a risk of liver damage even at recommended dosages, but the risk is increased with higher dosages, a shorter interval between doses, in people who drink three or more alcoholic drinks per day, when taken with other medications that also contain acetaminophen, and in patients with pre-existing liver disease.

5. Tips

• May be administered without regard to food; although food may decrease any reported stomach upset.
• Do not exceed the recommended dosage, because this may put you at risk of liver toxicity. Adults and teenagers who weigh at least 110 pounds (50kg) should not take more than 1000mg of acetaminophen in one dose or more than 4000mg in 24 hours. Children younger than 12 years should only take the dosage recommended to them on the label (this varies depending on their weight and age).
• Always seek your doctor's advice before administering acetaminophen to children aged less than two. If you are giving pediatric acetaminophen, always use the dosing syringe provided or another suitable properly calibrated measure. Do NOT use a kitchen teaspoon.
• Shake liquid acetaminophen before use. Chewable tablets should be properly chewed before swallowing. Hands should be dry before handling the acetaminophen disintegrating tablet, then the tablet should be placed on the tongue and allowed to fully dissolve before swallowing. Dissolve the disintegrating granules should be dissolved in at least four ounces of water; the mixture stirred, then drunk straight away.
• Be careful not to administer other products containing acetaminophen or paracetamol at the same time. Acetaminophen is often an ingredient in combination cold and flu remedies.
• Men should not drink more than two alcoholic drinks a day and women no more than one alcoholic drink per day while taking acetaminophen.
• Contact your doctor if symptoms worsen, you develop a skin rash, if redness or swelling occurs in the painful area, if your fever lasts longer than 3 days, or for persistent pain (excluding sore throat) of longer than 10 days in adults, 5 days in children and adolescents, or 3 days for infants.
• Contact your doctor immediately if you or your child have a severe sore throat or a sore throat that persists for longer than a few days, or is followed by fever, rash, headache, nausea, or vomiting.
• Seek urgent medical attention if you develop pain in your upper abdomen, dark urine, loss of appetite, yellowing of the skin or eyes, or clay-colored stools.
• Do not take acetaminophen without your doctor's advice if you are pregnant.

6. Response and effectiveness

• The pain-relieving effects of acetaminophen occur within 30-60 minutes of administration of the oral tablets. The effects last for three to four hours.

7. Interactions

Medicines that interact with acetaminophen may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with acetaminophen. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may have a moderate interaction with acetaminophen include:

• anticonvulsants, such as carbamazepine, fosphenytoin, or phenytoin (may increase the conversion of acetaminophen to hepatotoxic metabolites)
• barbiturates
• busulfan
• carbamazepine
• dapsone
• flucloxacillin
• isoniazid
• lamotrigine
• phenothiazines (may increase the risk of severe hypothermia)
• phenylephrine
• probenecid
• warfarin and other anticoagulants (may increase prothrombin time).

Alcohol may enhance the liver toxicity of acetaminophen. Avoid chronic use of alcohol or regular or excessive use of acetaminophen in people who drink excessively.

Note that this list is not all-inclusive and includes only common medications that may interact with acetaminophen. You should refer to the prescribing information for acetaminophen for a complete list of interactions.

References

• Acetaminophen. Revised 10/2024. ASHP monograph https://www.drugs.com/monograph/acetaminophen.html

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use acetaminophen only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

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