PrEP prevents HIV by putting antiretroviral medicine in the body before an exposure happens. If HIV then enters the bloodstream, the medicine blocks early steps the virus needs to establish a lasting infection. PrEP is highly effective when it is taken or received on schedule. It does not treat an existing HIV infection, and it does not prevent other sexually transmitted infections.
PrEP at a glance
- PrEP stands for pre-exposure prophylaxis: medication taken before there is a possible exposure to HIV.
- PrEP is intended for individuals who do not have HIV.
- Options include tablets and long-acting injections, although availability varies by country.
- You need to undergo an HIV test prior to commencing PrEP and again during follow-up.
- The best results come from consistent dosing and attending clinic appointments.
What happens when HIV enters the body?
HIV targets immune cells, especially CD4 cells. To establish an infection, it must enter a suitable cell, copy its genetic material and make more virus. Antiretroviral medicines used for PrEP interrupt that process early. When enough medicine is present in the tissues involved in an exposure, HIV is far less likely to gain a foothold.
This explains both why PrEP is such an effective preventive treatment and why it may not always be completely effective. Before exposure occurs, PrEP can provide significant protection. However, even though this protection exists, it is only present as long as there are adequate drug levels in the body. Missed tablets, delay of an injection, or beginning a regimen too close to the potential exposure can leave gaps in protection.
Your clinician or sexual health service can explain when a particular regimen becomes protective and how long it should be continued after the final possible exposure.
PrEP is intended for people without HIV. Anyone with a positive or uncertain result needs prompt specialist assessment because PrEP by itself is not complete treatment for established HIV.
How effective is PrEP?
The short answer is: very effective when used as directed. The CDC clinical guidance on PrEP estimates that oral and injectable PrEP reduce the risk of acquiring HIV through sex by about 99% when taken as prescribed. Oral PrEP also substantially reduces risk linked to injection drug use, although that evidence and effectiveness estimate differ from those for sexual exposure.
Those figures describe protection with good adherence; they are not a promise that every regimen is perfect. Breakthrough infections are rare, but they have occurred. Protection can fall when tablets are missed or injections are late. The route of exposure, the medicine used and how long it has been used all matter.
Randomised trials support modern PrEP practice. In HPTN 083, long-acting cabotegravir was superior to daily tenofovir disoproxil fumarate/emtricitabine among cisgender men and transgender women who have sex with men. The peer-reviewed HPTN 083 report also underlines a practical point: protection depends on keeping to the dosing schedule and testing carefully for HIV.
Which forms of PrEP are available?
Daily oral PrEP
Daily tablets are the most established form of PrEP. Most regimens combine emtricitabine with a form of tenofovir. The product a clinician recommends depends on local approvals, the type of exposure being prevented, kidney health, hepatitis B status, pregnancy considerations and other medicines.
Daily dosing can suit someone whose exposure is difficult to predict, and it removes the need to plan tablets around sex. The trade-off is simple: it requires a dependable routine.
Missing a dose of daily oral PrEP does not necessarily indicate failure of PrEP; however multiple missed doses over time may diminish efficacy. Individuals concerned about missing doses should consult with their provider for recommendations rather than guess.
Event-based oral PrEP
Some services offer event-based, on-demand or “2-1-1” oral PrEP to selected adults. It uses tablets before and after sex on a strict timetable. Research has focused mainly on gay and bisexual men taking tenofovir disoproxil fumarate/emtricitabine, so it is not suitable for every person or exposure route. Recommendations also vary by country. Event-based PrEP should be planned with a knowledgeable clinician, not improvised from leftover tablets.
Long-acting injectable PrEP
Long-acting cabotegravir is given on a schedule and can help people who would rather not take a daily tablet. More recently, twice-yearly lenacapavir has become another prevention option in some countries. The WHO guideline on lenacapavir for HIV prevention recommends offering it as an additional choice within combination HIV prevention programmes.
Long-acting does not mean “set and forget”. Clinic appointments and repeat HIV testing remain essential. Some injectable medicines stay in the body for months after stopping. If levels fall below the protective range during that tail period, a clinician may recommend another prevention method.
What needs checking before PrEP starts?
The first priority is confirming that the person does not have HIV. Depending on recent exposure, symptoms and previous PrEP or PEP use, this may involve a laboratory antigen/antibody test and an HIV RNA test. Starting PrEP during an unrecognised early infection could allow drug-resistant HIV to develop, which is why accurate testing matters.
A typical assessment for PrEP consists of:
- discussion of recent exposure and any symptoms compatible with acute HIV infection
- testing for HIV and other sexually transmitted infections
- kidney-function tests before tenofovir-containing oral PrEP
- hepatitis B testing before oral PrEP, because stopping tenofovir can reactivate hepatitis B
- review of pregnancy, contraception, other medicines and relevant health conditions
- a practical plan for adherence and follow-up
Symptoms cannot establish HIV status. Early infection may cause a flu-like illness, but many people have no symptoms, and the same symptoms occur in numerous common conditions. Testing is the only way to know.
How long does PrEP take to work?
There is no single timeframe for every regimen and exposure. With daily oral PrEP, drug levels associated with maximum protection are generally reached sooner for receptive anal sex than for receptive vaginal sex or injection drug use. Injectable products have their own starting schedules.
Ask the prescribing service when protection begins for the chosen regimen. If an exposure has already happened and adequate protection was not in place, urgent assessment for PEP may be needed. Our Infectious Diseases collection explains how the two approaches fit together.
Follow-up while taking PrEP
PrEP follow-up is part of the treatment plan, not an optional extra. Appointments are used for:
- Repeat laboratory testing for HIV
- Adherence evaluations
- Screening for other STIs.
Monitoring kidney function is necessary with certain oral regimens; scheduled appointment visits are mandatory with injectable formulations (with consideration given to delayed or missed injections).
Most side effects are mild and short-lived, though the pattern depends on the medicine. Oral PrEP can cause nausea, headache or stomach upset when first started. Tenofovir can affect kidney function in a small number of people. Injectable PrEP commonly causes temporary pain or tenderness at the injection site. Discuss new, severe or persistent symptoms with the prescribing clinician.
What PrEP does not do
PrEP does not prevent gonorrhoea, chlamydia, syphilis, hepatitis C or pregnancy. Condoms, vaccination where appropriate, sterile injecting equipment and regular sexual health testing still have a role. PrEP is one highly effective part of a broader prevention plan.
PrEP does not change the evidence behind “undetectable equals untransmittable”. A person living with HIV who takes effective treatment and maintains an undetectable viral load does not transmit HIV through sex. Someone may still choose PrEP for reassurance or for protection in other circumstances, but it is not needed to compensate for a partner’s sustained viral suppression.
When urgent advice is needed
Seek urgent medical care now if a possible HIV exposure occurred within the last 72 hours and PrEP was not being taken, had only just been started or may not have been used as prescribed. PEP is time-sensitive. Do not wait for symptoms or for the other person’s test result before asking for advice.
Frequently asked questions
Can PrEP give me HIV?
No. PrEP medicines do not contain HIV and cannot cause HIV infection.
Can I take PrEP only when I need it?
Some people can use a clinician-approved event-based oral schedule, but it is not suitable for every person or exposure route. Daily tablets or scheduled injections may be safer choices for others.
Do I still need HIV tests?
Yes. Testing before PrEP and at scheduled follow-up visits confirms that PrEP remains appropriate and helps detect a rare breakthrough infection promptly.
Can PrEP and PEP be used together?
They serve different moments. PEP is an emergency course after a possible exposure; PrEP provides continuing protection before future exposures. A clinician may arrange a direct transition from PEP to PrEP when ongoing protection is needed.
The practical conclusion
Understanding how PrEP prevents HIV comes down to timing and consistency: protective antiretroviral levels must be present before the virus can establish infection. Tablets and long-acting injections offer more than one way to achieve that protection. The best choice is the one that is medically suitable, locally available and realistic to keep using.
FMJ’s Medical Guides explain journal evidence and recognised clinical guidance for general education. This article cannot determine an individual’s HIV status or replace assessment by a sexual health or HIV clinician. Please read our medical disclaimer, or report a correction or editorial concern.