Drug Interactions and HIV/AIDS

Table of Contents

• Introduction
• How Drugs Are Metabolized
• Drugs That Slow Down Metabolism
• Drugs That Speed Up Metabolism
• Other Types of Drug Interactions
• Drugs and Other Substances That May Interact With HIV Medications
• Other Types of Drugs That May Interact With HIV Drugs
• Food
• Taking Care of Yourself

Introduction

In order for a drug to work properly, a person must take the correct dose so that the right amount of drug enters the bloodstream. Before an HIV drug is approved, researchers study different doses and choose one that is both safe and effective. The dose has to be high enough to stop HIV from making copies of itself, but not so high that it causes a lot of side effects.

All people living with HIV (HIV+) who are on treatment take more than one HIV drug, even if they only take one pill. Some pills contain more than one drug; for example, Truvada is a pill that contains the HIV drugs Emtriva (emtricitabine) and Viread (tenofovir). Many HIV+ people take other types of medications as well. Some prescription, over-the-counter, and recreational drugs, as well as herbs, vitamins, and supplements, can cause changes in the amount of HIV drugs in your bloodstream, even if you take the correct doses. Eating certain foods and beverages can also change drug levels in the blood.

When one drug affects the level of another drug it is called an interaction. Some drug interactions do not cause problems, but some interactions can be harmful. It is important to discuss the possibility of drug interactions with your health care provider when choosing a new HIV drug combination or when adding or removing any drug from your regimen.

How Drugs Are Metabolized

The body metabolizes (breaks down) the drugs you take. This process involves the liver and kidneys:

• Proteins in the liver called enzymes break down the drug
• The kidneys filter the drug out of the bloodstream and into the urine
• The drug is removed from the body in urine or feces

Sometimes, one drug affects the way another drug is metabolized by speeding up or slowing down 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.

Drugs That Slow Down Metabolism

Some drugs inhibit (slow down) the liver enzymes. This causes other drugs to be metabolized and eliminated from the system more slowly, which:

• Increases the amount of other drugs in the body
• Increases the length of time other drugs stay in the bloodstream

This can be useful in HIV therapy. Here is an example: Norvir (ritonavir) is a protease inhibitor (PI) that makes the liver enzymes work more slowly. This keeps some other drugs in the body longer and at higher levels. So if Norvir is given with another PI, like Reyataz (atazanavir), it "boosts" Reyataz. This means the amount of Reyataz in the blood is higher than it would be without Norvir.

As a result, one tablet of Reyataz can be taken once a day with a little Norvir instead of two tablets by itself. The boosted regimen increases the amount of Reyataz in the body and lowers the chance of developing resistance. Several other PIs can be boosted with Norvir. This can make the other PIs work better so that you can take lower doses and fewer pills.

Unfortunately, increased blood levels of drugs can also cause overdoses or increase side effects. If you are taking a drug that slows down liver enzymes, your health care provider may need to adjust the doses of your other medications.

Drugs That Speed Up Metabolism

Some drugs induce (speed up) the action of the liver enzymes. This causes other drugs to be metabolized and flushed out of the system more rapidly, which:

• Decreases the amount of other drugs in the body
• May cause other drugs to be less effective

Some drugs used to treat HIV-related conditions speed up liver enzymes. This can be a serious problem if it causes the HIV drugs to be metabolized too quickly. If HIV drug levels drop too low:

• HIV can make more copies of itself
• Viral load can go up
• Resistance can develop
• HIV drugs can stop working

The non-nucleoside reverse transcriptase inhibitors (NNRTIs), such as Viramune (nevirapine), speed up enzymes and get some other drugs out of the system more quickly. If you are taking a drug that speeds up liver enzymes, your health care provider may need to increase the doses of your other medications.

Other Types of Drug Interactions

If drugs cause similar side effects, combining them may increase the amount or seriousness of those side effects. For example, combining Zerit (stavudine) with Videx (didanosine) may increase the risk of a serious condition called lactic acidosis (high levels of lactic acid in the blood), especially in pregnant women. HIV+ pregnant women should not take Zerit and Videx together.

Some drugs work against each other and should not be taken together. An example is Retrovir (zidovudine) and Zerit.