WHAT ARE DRUG INTERACTIONS?

The amount of a prescription medication needs to be high enough to help fight a specific disease, and low enough to avoid causing too many side effects. Other medications, non-prescription (over-the-counter) drugs or recreational drugs, herbal products or even food sometimes cause large changes in the amount of a medication in your bloodstream. An overdose can cause serious side effects. An under-dose can mean that the medication will not work.

Everyone taking anti-HIV drugs needs to be very careful about drug interactions.

HOW DOES THE BODY PROCESS DRUGS?
Our body recognizes drugs as "foreign substances." It removes them, usually in urine or in bowel movements. Many drugs are removed unchanged by the kidneys. They leave the body in urine. Other drugs have to be processed by the liver. Chemicals in the liver (enzymes) change drug molecules. Then they are eliminated in urine or in bowel movements.

When you take a pill, the drug goes from the stomach into the intestine. Then it goes to the liver before it is circulated to the rest of the body. If the drug is easily broken down by the liver then very little of the drug reaches the body.

HOW DO DRUGS INTERACT?
The most common drug interactions involve the liver. Several drugs can slow down or speed up the action of liver enzymes. This can cause big changes in the blood levels of other drugs that are broken down by the same enzyme.

A few drugs slow down the kidneys. This increases the blood levels of substances that are normally removed by the kidneys.

The most common drug interactions involve the liver. Several drugs can slow down or speed up the action of liver enzymes. This can cause big changes in the blood levels of other drugs that are broken down by the same enzyme.

WHY DOES FOOD MATTER?
Any pills that you take go through the stomach. Most drugs are absorbed faster if the stomach is empty. For some medications, this is a good thing, but it can also cause more side effects. Some medications need to be taken with food so that they are broken down more slowly or to reduce their side effects. Other medications are taken with fatty foods because they dissolve in fat and are absorbed better.

Stomach acid breaks down some drugs, including ddI (didanosine, Videx®). The original ddI tablets include an antacid buffer that protects the drug from stomach acid. The buffer, however, interferes with the absorption of indinavir (Crixivan®), so these drugs should not be taken at the same time. Newer versions of ddI are easier to take.

WHAT DRUGS CAUSE THE MOST INTERACTIONS?
Protease inhibitors and non-nucleoside reverse transcriptase inhibitors are processed by the liver and cause many drug interactions.

Some other types of drugs that are likely to cause interactions include:

• Antifungal drugs with names that end in "-azole"
• Some antibiotics (names end in "-mycin")
• The antacid cimetidine (Tagamet®)
• Some drugs that prevent convulsions, including Dilantin® and Tegretol®.

• Some antihistamines
• Drugs to control heart rhythm
• Some pain killers derived from opium
• Propulsid®, which increases bowel activity
• Some sedatives, including Versed® and Halcion®
• Drugs to thin the blood, including Coumadin®
• Methadone
• Drugs to treat erectile dysfunction such as Viagra
• Some drugs used to treat tuberculosis, especially rifampin