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In some cases, using two drugs to treat an infection or a disease can be more advantageous than using only one drug. This is called combination drug therapy. One advantage of this method of treatment is that the dose of each drug can usually be reduced. This is important when the drugs being used have toxic side effects because using a lower dose of each drug will reduce side effects and toxicity. In addition, microorganisms are less likely to become resistant to drug treatment when two drugs are used simultaneously. Zidovudine (also called AZT) was the first drug that was shown to be effective in treating patients infected with the human immunodeficiency virus (HIV, which causes AIDS). However, recent studies have reported that the beneficial effects of zidovudine are limited, and that HIV may become resistant to zidovudine when it is used over long periods of time. Studies performed on cells grown in culture have shown that a combination of zidovudine and the drug foscarnet is more effective in killing HIV than either drug when used alone. Therefore, a study was performed to evaluate the effectiveness of combined therapy with zidovudine and foscarnet in six patients with HIV infection. The patients received treatment with zidovudine for 9 to 27 weeks prior to the beginning of the study. They received both zidovudine and foscarnet for two weeks, and were given zidovudine alone for the next six months. During the two-week period when both drugs were used, half the patients improved. However, 4 to 14 weeks later the beneficial effects of the combined treatment had disappeared. In treating HIV infections, it is possible that long-term treatment with both drugs will be required to maintain the beneficial effects of combination therapy. (Consumer Summary produced by Reliance Medical Information, Inc.)