Government approval of the first prescription pill to treat initial and repeat bouts of genital herpes and reduce frequent recurrences in some sufferers is expected to be announced today.

Researchers who have studied the drug, known as oral acyclovir, stress that it is not a cure for the sexually transmitted disease, which afflicts about 5 million to 20 million Americans. But it is thought to provide the first significant long-term relief for genital herpes sufferers, as long as the drug is taken on a daily basis.

Because the drug has the potential of preventing painful flare-ups of genital herpes, experts predict that it could become one of the most widely used drugs in this country. Studies have shown few side effects. However, both the Food and Drug Administration and the drug's manufacturer are expected to recommend that the drug be used for no more than six months at a time, until more long-term safety data is available.

The drug's approval by the FDA was scheduled to be announced at a news conference today by the manufacturer, Burroughs Wellcome Co. in Durham County, N.C. The FDA is also expected to release a statement. The company and the FDA yesterday declined to release approval and marketing details of the drug, which will be marketed under the trade name of Zovirax capsules.

Dr. Stephen Straus, a prominent herpes researcher at the National Institute of Allergy and Infectious Diseases, said yesterday that he is "enthusiastic" about the drug's use "in selected individuals."

But, he said, "I don't think it's a treatment for everybody who has genital herpes. I am cautious about being sure we watch for long-term effects. . . . My feeling is that the drug has been proven to be very safe . . . . In thousands of people in studies over the past four or five years, it's been very well tolerated."

Straus and others said that oral acyclovir, which keeps the genital herpes virus from growing but does not permanently kill it, was expected to be approved for the following uses:

* For treating first outbreaks of genital herpes, which are usually the most severe. In research studies, the drug was used five times a day for five to l0 days. "The disease is shortened by more than half, from an average of 2 1/2 weeks to about a week to a week and a half," said Straus.

* For treating recurrent bouts. In studies, it was used five times a day for five to seven days, shortening the outbreaks by "about 40 percent in duration and severity," he said, from about eight to about five days.

* For preventing or reducing frequent recurrences. Taken several times a day, it has been shown in studies that oral acyclovir can suppress 80 percent or more of recurrences, which may strike some people monthly. It may also reduce the spread of the disease to sex partners, but there is still no guarantee that an individual using the drug is not infectious at any given time.

Straus said, "When you're off, the effect of the drug disappears quickly . . . . "

"They're recommending courses up to six months in duration," he said. "There haven't been enough studies longer than six months to justify continuing," said Straus, who takes his patients off the drug for a time and may put some back on it to combat frequent recurrences.

He and others noted that there also is no data showing that the drug is safe for pregnant women.

Since 1982, a less effective ointment form of acyclovir has been sold by Burroughs for treatment of initial episodes of herpes. It is also available in intravenous form for hospitalized patients.