AZT extends the survival of some patients with AIDS, and both AZT and ddC are known to inhibit the growth of HIV. When AZT or ddC is given continuously over a prolonged period of time, toxic effects occur that are not found when the drugs are given for 4 - 6 weeks. It is hoped that by alternating the drugs or by giving one drug intermittently, the toxic effects can be decreased without lowering the therapeutic effectiveness of the drugs.