What is PrEP, and how does it work?
PrEP stands for pre-exposure prophylaxis. It uses antiretroviral medicine before a possible HIV exposure to stop the virus from establishing infection. PrEP is for people whose testing confirms that they do not have HIV. It is prevention, not a complete treatment regimen for someone who already has HIV.
CDC reports that prescribed oral and injectable PrEP reduce the chance of acquiring HIV through sex by about 99% when used as directed. Real-world protection depends on choosing an appropriate option, taking tablets consistently or receiving injections on schedule, and completing HIV testing and follow-up. Protection can decrease after missed doses, late injections, or an unplanned stop.
Who can consider PrEP?
Any person without HIV who wants additional protection can discuss PrEP with a qualified provider. A person does not have to accept a label such as “high risk.” PrEP may be particularly useful when condoms are not used every time, a partner’s HIV status is unknown, a person has multiple partners, has recently used PEP, has had an STI, or shares injection equipment.
A partner without HIV may also choose PrEP when their partner has HIV, especially if the partner’s current viral-load status is unknown or not consistently suppressed. A person with HIV who takes ART and maintains a viral load below 200 copies/mL will not transmit HIV through sex under U=U. Even then, the partner without HIV may request PrEP for autonomy, other partners, or additional reassurance.
What tests are needed before starting?
Confirming that a person does not have HIV is the most important safety step. Using an incomplete PrEP regimen when HIV is already present can allow drug resistance. A provider reviews the date of the last exposure, recent PrEP or PEP, and symptoms compatible with acute HIV. Ambiguous results may require an antigen/antibody test and HIV-1 RNA testing before PrEP is prescribed.
- HIV testing: selected according to exposure timing and recent antiretroviral use.
- Kidney function: required for certain oral PrEP medicines and monitored according to age and health.
- Hepatitis B: some oral PrEP medicines also treat HBV and should not be stopped without a plan.
- STI screening: syphilis, gonorrhoea, and chlamydia testing at relevant anatomical sites.
- Pregnancy and other medicines: discuss pregnancy plans, medical conditions, prescriptions, supplements, and allergies.
PrEP options and effectiveness
Options include daily oral PrEP and long-acting injectable PrEP. Availability depends on national registration, Thai clinical guidance, and the service. Daily oral PrEP has evidence across diverse populations. Injectable PrEP reduces the burden of daily tablets but requires on-time clinic visits and a safe plan if injections are discontinued.
Event-driven or “2-1-1” dosing uses F/TDF and has been studied only in adult gay and bisexual men. CDC describes it as off-label and does not recommend it as a general dosing approach. It is not appropriate for receptive vaginal exposure, active hepatitis B, or anyone unable to follow the timing reliably. Eligibility should be assessed under current Thai guidance rather than from an online schedule alone.
| Option | Potential advantage | Planning required |
|---|---|---|
| Daily oral PrEP | Continuous protection without predicting sex | Consistent tablets, HIV testing, and kidney monitoring when indicated |
| Long-acting injectable PrEP | No daily tablet | On-time injections and a transition plan when stopping |
| 2-1-1 for eligible adults | Linked to planned sexual activity | Confirm F/TDF, population eligibility, and the guidance used by the Thai service |
Follow-up while using PrEP
Follow-up is a safety system, not only a refill visit. It includes repeat HIV testing, adherence or injection review, side-effect assessment, kidney monitoring where appropriate, and STI testing based on exposure sites and needs. The schedule differs by PrEP type, age, health, and clinic protocol.
Tell the clinic before the next supply if you develop fever, rash, sore throat, or swollen lymph nodes after a recent exposure. Early nausea, headache, or stomach discomfort may occur and often improves, but do not stop without advice. If tablets were missed, an injection is late, or you want to stop, ask how long alternative protection is needed and how to stop safely.
What PrEP does not prevent
PrEP does not prevent syphilis, gonorrhoea, chlamydia, herpes, HPV, or pregnancy. Condoms and lubricant can reduce several STI risks. Site-specific screening can detect infections without symptoms, and hepatitis B or HPV vaccination may be appropriate.
PrEP use must never be treated as evidence of irresponsibility. It is proactive health care. Good services protect privacy, respect each person’s goals, avoid assumptions about identity or behaviour, and make prevention available without stigma.
Common PrEP questions
Does PrEP protect immediately?
Not in every situation. Time to protective drug levels differs by regimen and exposure site. Ask the prescriber how long to use an additional method.
Can PrEP make an HIV test positive?
No. PrEP does not create a positive result, but antiretroviral use can alter testing patterns in very early infection. Always report recent PrEP or PEP.
Can I stop PrEP on my own?
Plan discontinuation with the service, particularly after a recent exposure, with hepatitis B, or after long-acting injectable PrEP.
Making PrEP work in everyday life
PrEP is highly effective when it is used according to the plan agreed with a qualified provider. The right plan can depend on the product, sexual exposure pattern, kidney health, hepatitis B status, pregnancy, and how often protection is needed. Do not start, stop, or switch between daily and event-based dosing on your own. If you want to change, tell the service when you last took PrEP and when an exposure may occur so they can explain how long protection takes to build and how long dosing needs to continue afterward.
HIV testing before starting and at the recommended follow-up visits is essential because PrEP is intended for people who do not have HIV. Care may also include kidney monitoring, hepatitis B testing, and STI screening at the body sites that were exposed. A follow-up visit is more than a refill: it is a chance to review side effects, interactions with other medicines, missed doses, mental health, and additional options such as condoms and lubricant.
If you miss a dose, do not double or improvise a dose unless your clinic or the instructions for your specific regimen tell you to. If you have stopped PrEP and a possible HIV exposure occurred within the last 72 hours, contact a service urgently for a PEP assessment instead of waiting for symptoms or a test. After completing PEP, a provider can plan a direct transition to PrEP so there is no unintended gap in protection.
An STI result or a request for PrEP says nothing about a person’s worth or responsibility. Honest conversations about cost, work schedules, privacy, travel, and remembering medicine help a service find an option you can actually use. Phone reminders, linking a dose to an existing routine, keeping medicine in a safe and convenient place, and requesting refills before tablets run out can all make prevention more consistent.
PrEP does not interfere with the right to choose condoms, ask a partner about testing, or change your mind about sex. Some people use several strategies at once; others adjust their approach as relationships and circumstances change. The important thing is that the plan is medically appropriate, voluntary, and realistic. Tell the provider about persistent nausea, rash, reduced urination, severe illness, pregnancy plans, or difficulty taking doses so the cause can be assessed rather than simply stopping medicine without support.
Starting protection is not instantaneous for every regimen or exposure route. Ask the prescribing service when your specific product becomes protective, what to do before that time, and how long to continue after the last possible exposure. Event-based dosing is supported only for particular medicines and populations under appropriate guidance; it should not be improvised from a daily prescription. Injectable PrEP also has a scheduled dosing interval and requires a plan if an injection will be late.
Regular visits should include an opportunity to discuss acute HIV symptoms or a recent exposure before another prescription is issued. If HIV is suspected, the service may use additional testing and pause routine PrEP decisions until status is clear. This protects treatment options and prevents using an incomplete HIV regimen. Confidential, nonjudgmental care makes it easier to report missed doses and obtain the correct testing rather than hiding a problem.
Different PrEP products create different practical routines. Daily tablets need a dependable refill and a plan for days away from home. Long-acting injections require appointments on schedule and a backup plan if travel, illness, or clinic closure makes an injection late. If injectable PrEP is stopped, medicine can remain in the body at declining levels for months; the provider should assess continuing exposure, offer another effective method when needed, and arrange the recommended HIV tests during that period. Keep the product name, last dose or injection date, next appointment, and clinic number in a private record. If moving to another service, share those dates so the new team can continue protection safely rather than guessing. A good PrEP plan includes not only how to start, but also how to pause, restart, switch, or stop while preserving accurate HIV testing and future treatment options.
- Test for HIV before starting and at every recommended follow-up
- Tell the service about kidney conditions, hepatitis B, pregnancy, and all medicines or supplements
- Ask exactly when your regimen becomes protective and how long to continue after the last exposure
- Seek a PEP assessment within 72 hours if exposure occurs after protection has lapsed
- Arrange STI testing, condoms, lubricant, and contraception according to your needs
