News articles often describe Truvada, the daily medication that has been shown to greatly reduce the risk of contracting H.I.V., as controversial. But they tend to cite one specific Truvada opponent: Michael Weinstein, the outspoken president of the Los Angeles-based AIDS Healthcare Foundation.

Mr. Weinstein has called the use of Truvada to prevent H.I.V. — a practice known as pre-exposure prophylaxis, or PrEP — “a public health disaster in the making.” He has called Truvada a “party drug” and asserted that his loudest opponents on the issue have “all been associated with bareback porn.” Starting this week, the foundation is taking out advertisements in gay-oriented newspapers around the country under the headline, “What if you’re wrong about PrEP?”

Mr. Weinstein shows up in lots of news articles about Truvada in part because he runs a large H.I.V.-care organization, and in part because he gives colorful quotes. But he also shows up because he and the foundation stand more or less alone within the world of H.I.V.-prevention groups in their skepticism about PrEP.

“There’s no large controversy; there is one loud voice,” said Charles King, the president of the H.I.V. nonprofit Housing Works and a co-chairman of an anti-H.I.V. task force appointed by Gov. Andrew Cuomo of New York. Mr. King called the A.H.F. ad “a direct attack on New York State’s efforts to end AIDS as an epidemic.”