Acetylcysteine is used to help prevent or lessen liver damage caused by taking large quantities of acetaminophen (Tylenol).

There are other brands and forms of acetylcysteine available, and some can be purchased over the counter. Not all forms of acetylcysteine are effective as an acetaminophen antidote. Do not attempt to treat an acetaminophen overdose without medical advice.

Acetylcysteine may also be used for purposes not listed in this medication guide.

In an emergency, you may not be able to tell caregivers about your health conditions. Make sure any doctor caring for you afterward knows you received acetylcysteine.

Use exactly as directed on the label, or as prescribed by your doctor.

Do not take acetylcysteine at home if you don't understand all instructions.

If you are in an emergency medical setting: Before you take acetylcysteine, your caregivers will perform a blood test to measure the level of acetaminophen in your body. This test is most effective when performed within 4 to 8 hours after an acetaminophen overdose. If you cannot remember when you last took acetaminophen or how much you took, you will most likely be given the first dose of acetylcysteine right away.

Your doctor will determine how long to treat you with acetylcysteine. Do not stop taking acetylcysteine until your doctor tells you to.

If you vomit within 1 hour after taking acetylcysteine, you may need to take another dose.

Store at room temperature away from moisture and heat.

You should not use acetylcysteine if you are allergic to it.

If possible before you receive acetylcysteine, tell your caregivers if you have ever had:

• a stomach ulcer;

• stomach bleeding or bleeding in your esophagus (esophageal varices);

• high blood pressure, or if you are on a low-salt diet;

• congestive heart failure; or

• kidney disease.

Ask a doctor before using this medicine if you are pregnant or breastfeeding.

In an emergency, you may not be able to tell caregivers if you are pregnant or breast feeding. Make sure any doctor caring for your pregnancy or your baby knows you received this medicine.

Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

Follow your doctor's instructions about any restrictions on food, beverages, or activity.

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Acetylcysteine may cause serious side effects. Call your doctor at once if you have:

• severe or ongoing vomiting;

• coughing up blood or vomit that looks like coffee grounds; or

• signs that the medicine may not be working--upper stomach pain, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).

Common side effects of acetylcysteine may include:

• nausea, vomiting, upset stomach;

• rash; or

• fever.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Other drugs may affect acetylcysteine, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell your doctor about all your current medicines and any medicine you start or stop using.