Antiretroviral drugs
Antiretroviral drugs inhibit the growth and replication of HIV at various stages of its life cycle. Several classes of these drugs are available:

• Nucleoside analogue reverse transcriptase inhibitors (NRTIs). NRTIs were the first antiretroviral drugs to be developed. They inhibit the replication of an HIV enzyme called reverse transcriptase. They include zidovudine (Retrovir), lamivudine (Epivir), didanosine (Videx), zalcitabine (Hivid), stavudine (Zerit) and abacavir (Ziagen). A newer drug, emtricitabine (Emtriva), which must be used in combination with at least two other AIDS medications, treats both HIV and hepatitis B.

• Protease inhibitors (PIs). PIs interrupt HIV replication at a later stage in its life cycle by interfering with an enzyme known as HIV protease. This causes HIV particles in your body to become structurally disorganized and noninfectious. Among these drugs are saquinavir (Fortovase, Invirase), ritonavir (Norvir), indinavir (Crixivan), nelfinavir (Viracept), amprenavir (Agenerase), lopinavir (Kaletra) and atazanavir (Reyataz).

  The most common side effects of protease inhibitors include nausea, diarrhea and other digestive tract problems. PIs can also cause a significant number of side effects when they interact with certain other medications you may be taking. That's because all PIs, to one degree or another, affect an enzyme system in your liver that is responsible for metabolizing a large number of drugs. Other side effects have also appeared with the continuing and widespread use of protease inhibitors. These include elevated triglyceride levels and problems with sugar metabolism that may sometimes progress to diabetes.

  There may also be abnormalities in the way fat is metabolized and deposited in your body. Some people lose much of their total body fat; others gain excess fat on the back between their shoulders (buffalo hump) or in the stomach (protease paunch). No one knows exactly why these abnormalities occur. In fact it's not even certain whether these problems are a direct result of treatment with protease inhibitors or due to some other cause that has yet to be identified. Similar metabolic abnormalities have occurred in people on antiretroviral therapy that doesn't include PIs. Although these body changes can be distressing, the possibility they may occur should not stop you from obtaining treatment for HIV/AIDS.

• Non-nucleoside reverse transcriptase inhibitors (NNRTIs). These drugs bind directly to the enzyme reverse transcriptase. Three NNRTIs are approved for clinical use: nevirapine (Viramune), delavirdine (Rescriptor) and efavirenz (Sustiva). A major side effect of all NNRTIs is a rash. In addition, people taking efavirenz may have side effects such as abnormal dreams, sleeplessness, dizziness and difficulty concentrating.
  
• Nucleotide reverse transcriptase inhibitors (NtRTIs). NtRTIs work much like NRTIs: They interfere with the replication of reverse transcriptase and prevent the virus from inserting its genetic material into cells. But NtRTIs act more quickly than NRTIs do. The only approved drug in this class, tenofovir (Viread), inhibits both HIV and hepatitis B and appears to be effective in people who are resistant to NRTIs. The most common side effects of tenofovir, when used in combination with other antiretrovirals, are nausea, vomiting, diarrhea and gas. As with all reverse transcriptase inhibitors, the possibility of severe, and even fatal, liver damage exists.
  
• Fusion inhibitors. A challenging development in the AIDS epidemic is the emergence of drug-resistant strains of HIV. Worldwide, a majority of people receiving treatment for HIV are resistant to at least one drug, and many don't respond to a typical three-drug combination. But a drug called T-20 or enfuvirtide (Fuzeon), the first in a class of drugs called fusion inhibitors, appears to suppress even the most resistant strains of HIV. Fusion inhibitors stop the virus from replicating by preventing its membrane from fusing with the membrane surrounding healthy cells. Fuzeon is approved for use in combination with other HIV drugs and for adults and children older than 6 who have advanced infection and who have developed resistance to other drugs. Doctors administer Fuzeon by injection.

Your response to any treatment is measured by the levels of HIV in your blood (viral load). These levels should be tested at the start of treatment and then every three to four months while you're on therapy. In some cases you may be tested even more often.

New treatments
Many new drugs for HIV- or AIDS-related infections are in development or being tested in clinical trials. Among them are a drug that attacks the virus in the last stage of its life cycle and a pill that prevents the virus from entering cells, which researchers hope will avoid some of the toxic side effects of existing drugs. Although these medications are not yet licensed, some may be available by compassionate exception to people who need them.

Experts predict that an AIDS vaccine probably won't be found this decade. Trials of 30 vaccines are under way, but most of the vaccines are so similar that if one proves ineffective, the others will likely be ineffective too. One vaccine currently in clinical trails has already been shown not to work.

More promising is the search for a microbicide to protect women from HIV infection during sex. Although microbicides may never be as effective as condoms, which offer nearly 100 percent protection when used properly, a microbicide could still save millions of lives. Scientists think a drug that both kills the virus and protects vaginal tissue could be available within five to 10 years. One of the drugs being tested by the National Institutes of Health is a topical form of the NtRTI Viread.

For more information on new therapies, call AIDSinfo at (800) TRIALS-A, or (800) 874-2572. The National Library of Medicine's Health Services/Technology Assessment Text database also provides access to the full text of documents that may help you make treatment decisions.