A daily pill that can protect people from being infected by HIV is not as widely used as health officials would like, even though the drug has been available since 2012.

Pre-exposure prophylaxis (PrEP) — also known by the trade name Truvada — is a pill that contains the drugs tenofovir and emtricitabine.

When taken as directed, PrEP can reduce the risk of sexually acquired HIV infection by 90 percent or more. It can also reduce the risk of HIV infection among people who inject drugs by more than 73 percent.

Read More: Schooling Primary Care Doctors About HIV and PrEP »

Millions Could Benefit

The CDC estimates about 1.2 million people in the United States are at high risk of HIV infection and could benefit from PrEP and other effective prevention methods such as condoms and HIV treatment.

This high-risk group includes one in four sexually active gay or bisexual adult men, one in five adults who inject drugs, and one in 200 sexually active heterosexual adults.

In addition to these broad high-risk groups, health officials see other people who are ideal candidates for PrEP.

Dr. Scott Kim, medical director of HIV services at AltaMed Health Services in Los Angeles, told Healthline about one of his patients who is trying to get pregnant with her HIV-infected partner.

“For that couple, there really are no perfect options for conceiving in a safe way,” said Kim. “So she has elected to use PrEP so they can get pregnant together and start a family, but at the same time minimize her risk of contracting HIV.”

PrEP also provides an option for people who may be in relationships or social situations where they don’t have complete control, such as women in abusive relationships. They may be forced to have sex or forced to have sex without a condom.

“They are at serious risk of being infected in these relationships,” said Kim. “So they are certainly able to use PrEP to protect themselves.”

Read More: PrEP Seen as Big Part of White House HIV Strategy »

PrEP Use Is on the Rise

Between the beginning of 2012 and March 31, 2015, more than 8,500 people began using PrEP for HIV in the United States, said Ryan McKeel, spokesperson for Gilead Sciences Inc., the company that developed Truvada, in an email to Healthline.

This is based on prescriptions filled by 39 percent of the retail pharmacies in the United States that dispensed PrEP. It does not include demonstration projects that provide PrEP to participants directly.

While this is just a snapshot of PrEP use, said McKeel, “there was a 332 percent increase in the number of unique individuals that started Truvada for PrEP in the United States between first quarter of 2014 to the first quarter of 2015.”

Medicaid prescription data for New York State shows a similar jump in PrEP use.

Between July 2012 and June 2015, the number of New York State Medicaid participants starting PrEP increased more than fourfold.

Information from some major cities, though, shows that many high risk individuals are still not taking advantage of PrEP. Even in San Francisco, which has long promoted HIV prevention among its residents. In 2014, about 66 percent of HIV-negative bisexual or gay men met the criteria for PrEP, but only about 10 percent were actually using it.

In Philadelphia, around 90 patients are receiving PrEP from the four health centers that are part of the city’s PrEP program, said James Garrow, director of digital public health for the Philadelphia Department of Public Health, in an email to Healthline.

Read More: Though Rare, It’s Possible to Develop Drug-Resistant HIV While on PrEP »

Challenges of Wider Use of PrEP

Increasing the use of PrEP faces many challenges. Some people have difficulty accepting the “fact that maybe they would benefit from taking a pill every day to reduce their risk of HIV,” said Kim.

Lack of awareness of PrEP may also be keeping people from protecting themselves from HIV infection.

A recent study from Chicago in JAMA Internal Medicine found that among young African American men — one of the hardest-hit groups for new HIV infections — who have sex with men, only about 41 percent were aware of PrEP and less than 4 percent had used it.

Those who had a primary care provider were more likely to be aware of PrEP.

But that may not be enough. According to a 2015 CDC survey, more than one-third of primary care doctors and nurses don’t even know about PrEP. Which is why both the Centers for Disease Control and Prevention and the World Health Organization are pushing for broader use of PrEP.

People who are not seeking out PrEP may also be avoiding medical care.

“It has a bit to do with the fact that it’s intimidating for them to establish a relationship with a doctor in general,” said Kim, “And there are still lingering issues around healthcare coverage, whether they can afford to see a doctor.”