Of the 151 people studied, only 8 percent receive protease inhibitors, which rank high among the heavy hitters of AIDS medications. Only 28 percent take any anti-HIV drugs at all, the team found.

By contrast, almost 90 percent of middle-class San Franciscans infected with HIV receive medication for it, the scientists said.

"Poor people are increasingly the main high-risk group for HIV," said one UCSF researcher, Andrew Moss.

The study was run by Dr. David Bangsberg of the UCSF AIDS program at San Francisco General Hospital. It was presented at the 12th World AIDS Conference in Geneva.

The scientists also discovered a marked discrepancy between the patients' reported and actual compliance with drug-taking schedules. Failure to stick to the schedule might cause them to develop resistance to the medications, leaving them sicker than ever.

To double-check patient claims, the scientists installed a computer chip in the caps of bottles containing AIDS drugs. The chip recorded each time the bottle was opened.

The researchers found the patients were taking an average of only 80 percent of the pills. The patients claimed to have swallowed 92 percent. The discrepancy is significant: If a patient misses even a small number of drugs, his or her HIV level can soar.

Moss said the patients "had to take more than 90 percent of their medications to have a good chance of decreasing their viral loads to an undetectable level."

At the San Francisco AIDS Foundation, some HIV-infected clients skip medical appointments - especially those who don't feel sick. Some do so because they feel they must deal with more immediate day-to-day needs, such as locating food and shelter, said registered nurse Tim Teeter, who was not connected with either UCSF study.

Others who skip appointments include those whose judgment has been impaired by mental illness or substance abuse, Teeter said.

Some clients are reluctant to start on AIDS-fighting drugs such as protease inhibitors "because they're scared about the side effects," Teeter said. "All of these medications have side effects," Teeter explained. "If you've ever taken a course of antibiotics for a medication, you know it can cause an upset stomach. If you magnify that upset stomach by five to 10 times, you're talking about the kinds of effects that these kinds of (AIDS) drugs can cause."

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