Like the nucleoside analogs, NNRTIs block HIV's infection of new cells. Reverse transcriptase is a part of HIV required to make new virus and infect more cells. NNRTIs stop the reverse transcriptase from working properly.

In early clinical trials of these kinds of drugs, it was found that HIV developed resistance in as few as two to seven weeks when the drugs were used alone. The drugs are now being tested in combination with other drugs in an effort to prevent resistance and limit side effects seen with the nucleoside analogs.

Delavirdine (Rescriptor) is an approved NNRTI that has been shown in clinical trials to be well-absorbed by the body. Drug concentrations can be achieved in the blood that are 100 to 1,000 times the concentrations needed to inhibit HIV. So far, the major side effect seen with delavirdine has been a rash, which develops in 20 percent to 25 percent of patients. As with nevirapine, this side effect can be reduced by starting treatment at lower doses of the drug and escalating to a full dose.

Efavirenz (Sustiva) is the latest NNRTI to be approved. Sustiva does not seem to cause severe rash as often as other NNRTIs. The standard dose is 600 mg taken once daily. The main side effects are dizziness, anxiety, sleeplessness and altered mental state. These side effects seem most common during the first two weeks of treatment. Sustiva has not been studied for as long as other NNRTIs, but so far appears to have a very strong anti-HIV effect.

All NNRTIs are approved to be taken in combination with antiretrovirals. None should be taken alone. Some people are adding an NNRTI to a combination treatment that includes two antiretrovirals and a protease inhibitor. Another strategy is to use NNRTIs instead of a protease inhibitor. If you get a serious rash that does not go away in a few days you should stop taking the NNRTI immediately and contact your doctor.