Acetaminophen: Uses, Dosage, and Side Effects

Acetaminophen is an over-the-counter medication that relieves pain and lowers fever. The active ingredient in Tylenol, acetaminophen is a non-opioid analgesic (pain reliever) and an antipyretic (fever reducer). It is sold as tablets, capsules, gel caps, and liquid for infants and children.

Acetaminophen is also found in some combination cold medicines, migraine relievers, and prescription pain medication. While generally safe with few side effects, acetaminophen can be hard on the liver. Avoid exceeding the recommended dosage, and don't take it with alcohol.

This article discusses acetaminophen, its uses, and dosages. It also explains acetaminophen side effects, warnings, and potential interactions.

Uses

How acetaminophen works isn’t fully understood, but researchers suspect it has an effect on hormones called prostaglandins that cause pain and inflammation, as well as raise body temperature by affecting the hypothalamus in the brain.

Acetaminophen can be given to relieve nearly any sort of pain—headache, backache, toothache, menstrual cramps, body aches caused by colds and flu, injection-site pain from vaccinations, and more.

However, it is not an anti-inflammatory drug, so it does not help reduce swelling or inflammation, unlike Advil (ibuprofen) or Aleve (naproxen), which are common nonsteroidal anti-inflammatory drugs (NSAIDs).

Off-Label Uses

There is some evidence that acetaminophen may have beneficial effects on blood glucose levels and muscle function. Because it has antioxidant benefits, it may be useful for protecting the health of the heart and brain.

Acetaminophen is also sometimes used effectively in combination with aspirin and caffeine to relieve the pain associated with migraine headaches.

Before Taking

Acetaminophen often is the first treatment of choice for mild to moderate muscle aches, head pain and discomfort, or fever caused by colds. It’s imperative that you take it exactly as directed to avoid undesirable or even potentially life-threatening side effects.

Precautions and Contraindications

Generally speaking, acetaminophen is a safe medication, but there are precautions to keep in mind before taking it or giving it to a child:

• Don’t take acetaminophen if you have a history of liver disease.
• Keep track of the cumulative amounts of acetaminophen you’re taking or ask your pharmacist if you need help calculating it. If you use more than one product that contains acetaminophen, you may inadvertently exceed the maximum allowable daily dose, which can lead to serious side effects such as liver damage.
• If you’re giving acetaminophen to a child, read the package label carefully to make sure that it’s the right product for their age. Never give children acetaminophen products that are made for adults.
• Do not give acetaminophen to a baby under 12 weeks without your pediatrician’s guidance.
• Although acetaminophen is regarded as generally safe for a developing baby, if you’re pregnant or actively trying to conceive, check with your healthcare provider or OB-GYN before taking it.
• Be aware that acetaminophen can interact with certain medications. If you’re taking any of the following, check with your healthcare provider before taking acetaminophen: blood thinners such as Coumadin (warfarin) and Isoniazid (INH); seizure drugs such as Tegretol (carbamazepine), phenobarbital, and Dilantin (phenytoin); and phenothiazines (used as a tranquilizing drug and antipsychotic).
• If you have phenylketonuria (PKU), a genetic disorder that requires a diet low in foods that contain phenylalanine, it’s important to be aware that some brands of acetaminophen chewable tablets may be sweetened with aspartame, which is a source of phenylalanine.
• People who have compromised immunity, such as those with HIV/AIDS, are especially susceptible to acetaminophen poisoning and severe liver failure.

Dosage

The maximum allowable daily dose of acetaminophen for adults is 4,000 milligrams (mg). However, the Food and Drug Administration (FDA) suggests—but does not mandate—a maximum of 3,000 mg per day with no more than 650 mg every six hours, as needed. 

Because of concerns over liver damage from acetaminophen, manufacturers have erred on the side of caution in recent years. Johnson & Johnson, which produces Tylenol, has lowered the maximum daily dose for Tylenol Extra Strength, which contains 500 mg of acetaminophen, from eight pills per day (4,000 mg) to six pills per day (3,000 mg).

The dosing interval also has been changed from two pills every four to six hours to two pills every six hours. Regular Strength Tylenol contains 325 mg of acetaminophen per pill, which is the FDA-approved dosage.

If you take acetaminophen regularly and miss a dose, take the missed dose as soon as you remember it. If it’s almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Don’t take a double dose to make up for one missed.

Symptoms associated with acetaminophen overdose (whether accidental or not) include:

• Nausea
• Vomiting
• Loss of appetite
• Confusion
• Sweating
• Extreme fatigue
• Unusual bleeding or bruising
• Stomach pain (especially on the upper right side)
• Yellowish skin or eyes
• Flu-like symptoms
• Diarrhea
• Irregular heartbeat

If you or someone else develops any of these symptoms, medical help should be sought immediately.

All listed dosages are according to the drug manufacturer. Check your prescription and talk to your healthcare provider to make sure you are taking the right dose for you.

Modifications

The dosage of acetaminophen for children is based on weight. The maximum recommended daily dose for those under 12 and/or who weigh less than 50 kilograms (kg), which is around 110 pounds, is 75 mg/kg (milligram of medication per kilogram of body weight), or 10 to 15 mg/kg every four to six hours as needed, and no more than five doses per 24-hour period.

How to Take and Store

Acetaminophen can be taken with food or on an empty stomach. However, do not take it if you’ve had more than three alcoholic beverages in a day.

If you’re taking extended-release acetaminophen tablets, swallow them whole—do not split, chew, crush, or dissolve them.

Shake liquid acetaminophen before each use to mix the medication evenly. Use the measuring cup or syringe provided by the manufacturer to measure each dose of the solution or suspension.

Keep acetaminophen products tightly closed in the containers they came in, out of reach of children. Store bottles at room temperature and away from excess heat and moisture (not in the bathroom).

Side Effects

As with most medications, acetaminophen carries the potential for side effects ranging from relatively minor to serious and even life-threatening.

Common

Two common side effects of acetaminophen which don’t usually require medical attention are:

• Headache
• Nausea

Severe

More serious adverse reactions associated with acetaminophen include:

• Allergic reaction, such as rash, itching, hives, and swollen face, lips, or tongue
• Sore throat with fever, headache, nausea, rash, or vomiting
• Swelling
• Hoarseness
• Difficulty breathing
• Difficulty swallowing
• Stevens-Johnson syndrome and toxic epidermal necrolysis—potentially life-threatening skin reactions that typically require hospitalization
• Acute generalized exanthematous pustulosis, a less serious skin reaction that usually resolves within two weeks after stopping acetaminophen

Call your healthcare provider immediately or go to an emergency room if you experience any of these side effects.

Warnings and Interactions

Products with acetaminophen contain warnings regarding severe liver damage, overdose, and allergic reaction.

In 2011, the FDA asked acetaminophen manufacturers to limit the strength of acetaminophen in prescription drugs to 325 mg per pill.

Products containing acetaminophen also started carrying the FDA’s strongest “black box” warning label highlighting the potential for serious liver injury and allergic reactions (e.g., swelling of the face, mouth, and throat; difficulty breathing; itching; or rash).

Printed in red on the cap of Extra Strength Tylenol are the words “Contains Acetaminophen” to remind consumers of the potential harmful effects of excessive amounts of this drug.

Studies have found acetaminophen overdose to be the leading cause of acute (sudden) liver failure in the United States. The risk is highest when large doses of acetaminophen are taken or the medication is used frequently over a long period time.

Taking acetaminophen with alcohol or certain drugs associated with liver damage, such as the cancer treatment Gleevec (imatinib) and the tuberculosis drug Tubizid (isoniazid), also can increase the risk of acute liver failure.