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Q: I take about four extra-strength Tylenol most days. Do I need to worry about my liver?

A: Acetaminophen, the active ingredient in Tylenol (and other brand names), is widely considered to be a very safe medication. For the vast majority of those taking it, acetaminophen appears to have no harmful effect on the liver or other parts of the body.

Still, your question is a good one because, although its safety record is quite good, too much acetaminophen — whether accidental or intentional — is a leading cause of liver failure requiring a transplant.

How can this be? One reason is that it’s easy to take too much. Acetaminophen is widely available without a prescription, and it’s contained in many combination medications — more than 600, according to one estimate — such as Nyquil, Excedrin and Midol.

It’s likely that many people taking acetaminophen do not realize they’re taking too much. For example, you might take acetaminophen tablets for the achy feeling that comes with a cold and also use a combination cough medicine that contains acetaminophen.

In addition, the reputation of acetaminophen as a safe drug may lead people to figure “more is better” and that a higher-than-recommended dose won’t cause trouble.

The maximum recommended total dose of acetaminophen is 3,000 milligrams (mg) per day. Each extra-strength acetaminophen dose is 500 mg. So, that means no more than six extra-strength Tylenol over 24 hours, taking no more than two pills at a time.

Riskier for some people

But these recommendations apply to otherwise healthy people. Those who already have liver disease or risk factors for liver disease are generally advised to use acetaminophen sparingly in smaller doses or avoid it entirely.

People who drink more than moderate amounts of alcohol or already have liver disease are more likely to develop liver damage from acetaminophen.

For example, a person with a hepatitis C infection may be prone to liver damage when taking doses of acetaminophen that might be harmless to others.

Risk is also higher for people who use combination acetaminophen-opiate products (such as Percocet). Increasing the dose of the opiate to control pain may provide too much acetaminophen.

Intentional overdose of acetaminophen in an attempted suicide can result in liver failure if the person doesn’t seek emergency care.

Acetaminophen is a good example of how almost any medication can cause serious side effects — just because it’s available over the counter doesn’t mean it can’t cause trouble. Read the label of any medication you take and make sure you aren’t doubling up on acetaminophen.

Acetaminophen is safe and effective for most people who take it. Liver damage related to acetaminophen is highly preventable.

Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, see health.harvard.edu.

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