How HIV PrEP Works: From Daily Pills to 6-Month Shots

The Drug That Stops HIV Before It Starts

For most of the HIV epidemic's history, prevention meant behavior change — condoms, clean needles, and awareness campaigns. Then came a medical breakthrough: a pill that, taken consistently, could stop HIV from ever taking hold in the body. That strategy is called pre-exposure prophylaxis, or PrEP — and it has quietly become one of the most effective tools in modern infectious disease prevention.

Recent advances have pushed PrEP even further, from a daily pill to a twice-yearly injection that protects against HIV with near-total effectiveness. Understanding how these drugs work illuminates not just HIV science, but the broader logic of antiviral medicine.

What PrEP Actually Is

PrEP is a preventive medication regimen taken by people who do not have HIV but are at risk of acquiring it — through sex or injection drug use. Unlike a vaccine, PrEP does not train the immune system. Instead, it floods the body with antiviral drugs that block HIV from completing its replication cycle if the virus is encountered.

According to the U.S. Centers for Disease Control and Prevention (CDC), consistent PrEP use reduces the risk of acquiring HIV through sex by approximately 99% and through injection drug use by at least 74%. The key word is "consistent" — effectiveness drops sharply when doses are missed.

How HIV Infects the Body

To understand PrEP, it helps to know how HIV works. The virus targets a specific type of immune cell — the CD4+ T-cell — which plays a central role in coordinating the body's immune response. HIV attaches to receptors on the cell's surface, injects its genetic material, and hijacks the cell's machinery to produce copies of itself. Those copies burst out, infect new cells, and gradually destroy the immune system over months or years.

This life cycle has several distinct stages: entry into the cell, reverse transcription (converting viral RNA into DNA), integration into the host genome, assembly of new viral particles, and release. Antiretroviral drugs — the same drugs used to treat HIV — can interrupt any of these stages.

How the Classic PrEP Pill Works

The original oral PrEP, marketed as Truvada, combines two drugs: tenofovir and emtricitabine. Both belong to a class called nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs). They work by mimicking the building blocks that HIV uses to copy its genetic code — but when the virus incorporates these impostors, replication halts.

The iPrEx trial, the landmark 2010 study supported by the National Institute of Allergy and Infectious Diseases, first demonstrated that daily oral PrEP could dramatically reduce HIV acquisition in people at high risk. The FDA approved Truvada for PrEP in 2012, marking a turning point in prevention medicine.

The Leap to Injectable PrEP: Lenacapavir

The newest generation of PrEP works in an entirely different way. Lenacapavir (brand name Yeztugo in the U.S.) is the world's first capsid inhibitor — and it is administered just twice a year by injection.

HIV's capsid is a protein shell that surrounds the virus's genetic material and plays a role in nearly every stage of the viral life cycle: entry into the nucleus, assembly of new particles, and release from infected cells. Lenacapavir binds directly to the junction between capsid protein subunits, disrupting all of these processes simultaneously. As researchers at the National Institutes of Health and Gilead Sciences have detailed, most antivirals block one step; lenacapavir blocks several, which also makes it active against strains resistant to other drug classes.

In the PURPOSE 2 clinical trial, twice-yearly lenacapavir injections resulted in a 99.9% HIV-free rate among participants — only two infections among 2,179 people in the lenacapavir group. The FDA approved it for HIV prevention in June 2025.

Who Should Consider PrEP?

The World Health Organization recommends PrEP for anyone at substantial risk of HIV infection, including people with an HIV-positive partner, people who do not consistently use condoms with partners of unknown status, and people who inject drugs and share equipment. It is not a lifetime commitment — PrEP can be started, paused, or stopped based on changing circumstances.

Access and the Global Challenge

Lenacapavir's effectiveness is extraordinary, but so was its original price — around $40,000 per patient annually in the United States. In response to global pressure, Gilead Sciences struck licensing agreements with generic manufacturers to supply the drug for approximately $40 per year in 120 low- and middle-income countries, with first shipments reaching Zambia and Eswatini in 2025.

Closing that access gap remains the central challenge in HIV prevention: science has provided the tool; health systems and political will must do the rest.