What Is PrEP for HIV? How It Works, Who It's For, and How to Get It

How does PrEP work?

PrEP contains antiretroviral medications that, when maintained at steady levels in the body, interfere with a key step in HIV's replication cycle. When drug levels are consistently high enough — achieved by taking it exactly as directed — the window during which HIV could establish an infection is essentially closed 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence.

This differs from post-exposure prophylaxis (PEP), which is taken *after* a potential exposure. PrEP is planned, ongoing prevention; PEP is an emergency intervention. If you had a possible HIV exposure in the last 72 hours and are not currently on PrEP, PEP — not PrEP — is the relevant option.

Who is PrEP recommended for?

The CDC's 2021 PrEP clinical practice guideline recommends PrEP for HIV-negative people at substantial risk of HIV 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence. Clinicians consider factors including:

• A sexual partner living with HIV (especially if not on fully suppressive treatment)
• Inconsistent condom use during sex with partners of unknown or positive HIV status
• A recent bacterial STI — such as gonorrhea or syphilis — which signals high-risk sexual activity
• Injecting drugs and sharing equipment
• Personal desire for an additional layer of protection beyond condoms

PrEP is appropriate for anyone at elevated risk, regardless of sex, gender, or sexual orientation. The USPSTF issued a Grade A recommendation in 2023 that clinicians prescribe PrEP to people at increased risk of HIV acquisition 2Ref 2US Preventive Services Task Force; Davidson KW, Barry MJ, Mangione CM, et al. (2023).Preexposure Prophylaxis to Prevent Acquisition of HIV: US Preventive Services Task Force Recommendation Statement.USPSTF Grade A recommendation (2023) that clinicians prescribe PrEP to people at increased risk of HIV acquisition; coverage of injectable cabotegravir and oral TAF/FTC in addition to TDF/FTC.

What are the PrEP options?

There are several approved approaches; a clinician will determine which fits your situation 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence:

Daily oral PrEP: One tablet taken every day. This is the most widely available form and works across all exposure types — sexual and injection-related.

On-demand oral PrEP: Taken before and after specific sexual encounters rather than daily. This approach has been studied specifically in cisgender men who have sex with men for anal sex exposures. It is not recommended for people who inject drugs or for vaginal-sex exposures.

Long-acting injectable PrEP: An injection given every two months by a clinician. This option removes the daily pill requirement and may suit people who find daily adherence difficult 2Ref 2US Preventive Services Task Force; Davidson KW, Barry MJ, Mangione CM, et al. (2023).Preexposure Prophylaxis to Prevent Acquisition of HIV: US Preventive Services Task Force Recommendation Statement.USPSTF Grade A recommendation (2023) that clinicians prescribe PrEP to people at increased risk of HIV acquisition; coverage of injectable cabotegravir and oral TAF/FTC in addition to TDF/FTC.

Before starting any form of PrEP, a clinician must confirm you are HIV-negative — starting PrEP in someone who already has HIV can contribute to drug resistance 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence.

How effective is PrEP?

PrEP is highly effective when taken as directed 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence2Ref 2US Preventive Services Task Force; Davidson KW, Barry MJ, Mangione CM, et al. (2023).Preexposure Prophylaxis to Prevent Acquisition of HIV: US Preventive Services Task Force Recommendation Statement.USPSTF Grade A recommendation (2023) that clinicians prescribe PrEP to people at increased risk of HIV acquisition; coverage of injectable cabotegravir and oral TAF/FTC in addition to TDF/FTC. Effectiveness depends heavily on adherence — missed doses reduce protection. PrEP does not protect against other STIs (gonorrhea, chlamydia, syphilis, herpes), which is why regular STI screening every three months is built into PrEP care 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence. Screening also includes HIV testing, kidney function labs, and prescription refills.

What does PrEP care involve — including cost and side effects?

Ongoing PrEP care typically includes an HIV test, kidney function labs, and STI screening every three months, plus prescription refills 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence. This built-in monitoring makes PrEP care a model for integrated sexual health.

Side effects: Most people tolerate PrEP well. Some experience mild nausea, headache, or fatigue in the first few weeks that often resolves. Some formulations can affect kidney function or bone density over time, which regular monitoring tracks. Serious side effects are uncommon 1Ref 1Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.PrEP mechanism of action; eligibility criteria (partner with HIV, recent STI, inconsistent condom use, injection drug use); daily oral, on-demand, and injectable formulations; need to confirm HIV-negative status before starting; quarterly monitoring including HIV test, kidney function, STI screening; side effect profile; Ready Set PrEP program; effectiveness dependent on adherence.

Cost and access: The federal 'Ready, Set, PrEP' program provides PrEP free to people without insurance. Most Medicaid and many private insurance plans cover it 3Ref 3US Preventive Services Task Force; Owens DK, Davidson KW, Krist AH, et al. (2019).Screening for HIV Infection: US Preventive Services Task Force Recommendation Statement.ACA coverage context for HIV-related preventive services and PrEP; USPSTF recommendation framework for HIV prevention that underpins insurance coverage requirements. A clinician or patient navigator can identify the right coverage path.