The ads are from law firms seeking to recruit gay and bisexual men for a class-action lawsuit against Truvada maker Gilead Sciences, NBC News reported Wednesday.
Truvada - a type of pre-exposure prophylaxis (PrEP) - is a once-a-day pill that is 99% effective at preventing HIV transmission, the U.S. Centers for Disease Control and Prevention says.
"The advertisements are targeting LGBTQ Facebook and Instagram users, and are causing significant harm to public health," the letter states. "The law firms' advertisements are scaring away at-risk HIV-negative people from the leading drug that blocks HIV infections," the letter states.
The groups say the ads are misleading because they warn against side effects that mainly occur with long-term treatment for people who already have HIV, NBC News reported.
Facebook values its "work with LGBTQ groups" and both welcomes and seeks out their input, a company spokesperson said in a statement sent to NBC News.
"While these ads do not violate our ad policies nor have they been rated false by third-party fact-checkers, we're always examining ways to improve and help these key groups better understand how we apply our policies," the spokesperson said.
The class-action lawsuit claims that certain side effects among some patients taking Truvada could have been prevented if Gilead had not delayed the release of a safer version of the original drug, which was shelved in 2004, NBC News reported.
Only about 18 percent of the 1.2 million Americans who might benefit from using Truvada got a prescription for it last year, a recent CDC report found.