How Drug Resistance Occurs

• How Does Resistance Affect Your Treatment Options?

HIV medications are best at fighting the regular ("wild-type") -- not the mutated -- virus. Although most HIV mutations are harmless, sometimes HIV can get lucky: If a mutation differs in just the right way, it can render an HIV medication useless. If this happens, we say that HIV is "resistant" to that medication. Suppose you take your medications erratically -- meaning you frequently miss a dose. HIV will grab this opportunity to start making more copies of itself in your CD4 cells. As the amount of HIV in your body increases while you are not taking your medications consistently, so does the amount of mutated HIV -- including mutations that make your virus resistant to your medications.

In other words, if you miss doses of your medications too often, you're effectively training your mutated, drug-resistant HIV to survive better. The mutated virus may begin to make more and more copies of itself, and could eventually become the most common type of HIV in your body. Once this happens, no matter how much of that medication you take, it will no longer have an effect on your HIV. This means you've become resistant to that medication. Once your HIV has developed resistance to a medication, it will stay resistant forever, since resistant HIV, like wild-type HIV, can remain hidden in some of your cells.

If you often take your medications inconsistently, HIV will have a chance to make more copies of itself. Many of these new copies may have mutations, some of which might be resistant to one or more of your HIV medications.

How Does Resistance Affect Your Treatment Options?

If you do not take your medications on time, HIV mutations can make up a larger and larger percentage of the HIV in your body, and the odds of developing drug resistance will greatly increase.

Some HIV mutations, as we've mentioned, are more dangerous than others. Some will cause resistance to just one drug. But other HIV mutations can cause resistance to two or more drugs -- or even to an entire class of drugs.

This is especially true for the class of drugs we call NNRTIs. Just one specific type of mutation will make HIV resistant to all NNRTIs: efavirenz (Sustiva), nevirapine (Viramune) and delavirdine (Rescriptor). This is called "cross-resistance" because the drug resistance "crosses over" from one drug to another, in the same way a bug that's immune to one bug spray might also be immune to other bug sprays.

Sometimes, though, it takes more than one mutation for HIV to become resistant to a drug. This is true for several HIV meds, particularly some members of the protease inhibitor class.

Resistance to a drug does not automatically mean you can no longer use it. There are different degrees of resistance. Sometimes the medication will still work, just not as well as it used to. How will you know if it's still worth taking a drug that your HIV has become resistant to? There are many factors for you and your doctor to consider, including which HIV medications you've taken in the past, which specific medications your HIV is resistant to, your current CD4 count and viral load, and which medications you haven't tried yet.