Truvada is a drug used to treat HIV but was approved earlier this year for those at high risk of becoming infected with the virus that leads to AIDS.
The new research from the University of California, San Francisco was designed to learn more about patient adherence to HIV medication.
Truvada is supposed to be taken daily, but researchers found the drug was highly effective even when patients skipped days.
Still, doctors stress it's important to adhere to the daily regimen, and use condoms for stronger protection.
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