It is now clear that the most effective way of using anti-HIV drugs is in combination. In the autumn of 1995, the results of the Delta trial and ACTG 175 proved that combinations of AZT (zidovudine, Retrovir) plus ddI (didanosine, Videx) or ddC (zalcitabine, Hivid) were more effective than AZT alone at delaying disease progression and prolonging life.

Other studies demonstrate that triple-drug combinations consisting of two nucleoside reverse transcriptase inhibitors plus either a protease inhibitor or a non-nucleoside reverse transcriptase inhibitor have better anti-HIV effects than two drug regimens.

There are two main reasons which explain why combination therapy is more effective:

• Different drugs work in different ways, or attack the virus at varying stages of its lifecycle. Combining such drugs allows the virus to be countered on a number of different fronts, increasing the antiviral effects.

• Simultaneous treatment with a number of different drugs may make it harder for the virus to replicate and thereby develop resistance to one or all of the drugs.

Combination therapy is discussed in more detail in When to start treatment, Changing treatment and Choosing a combination in Anti-HIV therapy.