An executive from CVS has demanded Gilead lower the prices of a new HIVprevention drug in an email .

Dr. Michelle Guardine, chief medical officer for CVS Caremark, sent an email to PrEP4All, an organization working on PrEP access, that has demanded the pharmacy chain cover the new drug Yeztugo, also known as lenacapavir. In the message, which was posted to social media, the executive said the reason CVS controversially refused to cover the highly effective pharmaceutical was a concern over costs.

“We believe that easy access to PrEP medications is critical for Americans who may be exposed to HIV,” Gourdine wrote. “Our formularies cover several PrEP options, both injectable and oral. For drugs excluded from our standard formularies, exceptions are available when medically justified.”

But she said Gilead has priced Yeztugo at $28,000 within the U.S., even though generic versions of lenacapavir will be sold in other countries for $40.

“It is clear, Gilead can lower its price in the U.S. for Yeztugo, and we continue to call upon them to do so,” the email reads. “It is inappropriate for branded pharmaceutical manufacturers to try to manipulate pre-existing guidelines with clinically similar products that are priced far higher than what’s already on the market.”

That is why CVS Health has not added Yeztugo to its list of recommended PrEP therapies, but the company will cover alternatives, according to Gourdine.

“In increasingly crowded therapy classes of highly effective options, a generics-first policy remains the best approach for affordability and, by extension, health outcomes,” Guardine wrote. “We will continue to urge Gilead to lower the U.S. price for Yeztugo.”

The Centers for Disease Control and Prevention in September issued guidelines allowing twice-a-year injections for Yeztugo, and found near 100 percent effectiveness in trials for patients remaining HIV negative. Gilead has touted the drug as a major medical advance.

“Yeztugo represents a transformative scientific advancement in HIV prevention—delivering high efficacy across broad, representative populations in clinical trials and approved as the first and only twice-yearly injectable option in the U.S.,” said Dr. Dietmar Berger, chief medical officer at Gilead Sciences.

“As we strive to reimagine the future of HIV prevention, the CDC’s endorsement of Yeztugo offers healthcare providers, public health leaders and communities clear guidance on an innovation that could help shift the trajectory of the HIV epidemic.”