From POZ Magazine online…
Drug resistance refers to the ability of viruses or bacteria to continue multiplying despite the presence of drugs that usually disable or kill them. In the case of HIV, drug resistance occurs when the virus develops mutations, or changes in its genetic code. These changes alter HIV proteins—including the reverse transcriptase, protease and integrase enzymes the virus uses to replicate—in ways that reduce susceptibility to drugs.
Inadequately treated HIV replicates rapidly and can quickly develop mutations that confer drug resistance. Sometimes resistance mutations can make an entire class of drugs ineffective. Certain antiretroviral classes have a higher barrier to resistance, meaning they are less likely to stop working. For example, some meds require more mutations to compromise their effectiveness, and others bind to the virus in such a way that they can keep working even as the virus mutates. As a class, non-nucleoside reverse transcriptase inhibitors (NNRTIs) have a lower barrier to resistance, while protease inhibitors and integrase inhibitors have a higher barrier to resistance.