PrEP stands for pre-exposure prophylaxis. It is a medication taken by HIV-negative individuals to prevent HIV infection. When taken correctly, it is highly effective — reducing the risk of acquiring HIV through sex by approximately 99%.[1] It is one of the most significant advances in HIV prevention of the last two decades, and it remains underused.
This article explains what PrEP is, who it is for, how it works, and what starting it involves in Singapore.
How PrEP works
PrEP works by maintaining a level of antiretroviral medication in the body’s tissues that prevents HIV from establishing infection if the virus is encountered. The medication does not treat HIV — it prevents it from taking hold.
The standard formulation in Singapore is a combination of two antiretroviral drugs: tenofovir and emtricitabine, taken as a single tablet. Two versions are available — tenofovir disoproxil fumarate with emtricitabine (TDF/FTC), and tenofovir alafenamide with emtricitabine (TAF/FTC). Both are approved by Singapore’s Health Sciences Authority (HSA) and are highly effective; TAF/FTC has a more favourable kidney and bone profile and is generally preferred for men with relevant risk factors.[1]
Daily PrEP and on-demand PrEP
PrEP can be taken in two ways, and the right choice depends on the individual’s pattern of sexual activity.
Daily PrEP involves taking one tablet every day, regardless of sexual activity. It builds a consistent protective level of drug in the body’s tissues. The PROUD trial, a randomised controlled trial conducted in England, demonstrated an 86% reduction in HIV incidence in men who have sex with men using daily PrEP compared with deferred treatment.[2] Daily PrEP is the most straightforward regimen and is appropriate for men who are sexually active regularly or unpredictably.
On-demand PrEP — also called event-driven PrEP or the 2-1-1 regimen — involves taking PrEP around specific sexual encounters rather than daily. The schedule is: two tablets taken two to twenty-four hours before sex, one tablet twenty-four hours after the first dose, and one tablet forty-eight hours after the first dose. The IPERGAY trial, a randomised controlled trial conducted in France and Canada, showed an identical 86% reduction in HIV incidence with this regimen compared with placebo.[3]
On-demand PrEP is suitable for men who have sex infrequently, at planned times, and who can reliably follow the dosing schedule around each encounter. It requires more adherence attention than daily PrEP and is not recommended for men whose sexual activity is frequent or unpredictable. It is also important to note that on-demand PrEP has only been studied and validated in men who have sex with men — the evidence base for other populations uses the daily regimen.[1]
Who should consider PrEP
PrEP is not for everyone — it is a targeted prevention tool for individuals at meaningful, ongoing risk of HIV exposure. In Singapore, candidates typically include:
- Men who have sex with men with multiple partners or inconsistent condom use
- Men in relationships where one partner is HIV-positive
- Men who have recently had a sexually transmitted infection, which indicates higher-risk sexual behaviour
- Men who use recreational drugs in sexual contexts (chemsex)
- Heterosexuals who have more than 1 sexual partner and have casual partners (partners who they cannot contact again) should also be on PrEP. This is because there is no way to determine if those partners are HIV-positive if the individual has not managed to establish their casual partner’s HIV status before engaging with them.
PrEP is not appropriate for individuals who are already HIV-positive, who have a contraindication to the medication (such as significantly impaired kidney function). Whether you are a suitable candidate is determined at consultation, not assumed in advance.
If you are unsure whether you fall into a higher-risk category, that is exactly the question for a doctor.
What starting PrEP involves
Before starting PrEP, a baseline assessment is required. This involves:
- An HIV test — you must be confirmed HIV-negative before starting. If there has been a recent high-risk exposure in the last 72 hours, PEP (post-exposure prophylaxis) may be needed first, before PrEP is started.
- Kidney function tests, as tenofovir-based PrEP requires adequate renal function
- Hepatitis B status — TDF/FTC has activity against hepatitis B, which has implications for stopping the medication
- A broader STI screen is also recommended at baseline and at each follow-up
Once on PrEP, monitoring every three months is standard practice: HIV testing, kidney function, and STI screening at each visit.[1] The monitoring is not a formality – it is the part that keeps PrEP safe. A doctor who prescribes PrEP should be managing the follow-up as part of the same programme.
What PrEP does not protect against
PrEP is highly effective against HIV. It does not protect against other sexually transmitted infections — chlamydia, gonorrhoea, syphilis, herpes, HPV, and hepatitis C are all transmitted independently of HIV and are not prevented by PrEP. Condoms reduce the risk of these infections and remain a sensible part of a broader sexual health strategy, particularly for men on PrEP who may be having sex with multiple partners.
PrEP and PEP — the difference
PrEP is taken before exposure, on an ongoing basis, as prevention. PEP (post-exposure prophylaxis) is taken after a potential HIV exposure, as an emergency intervention. PEP must be started within 72 hours of exposure — the sooner the better — and runs for 28 days. If you think you may have been exposed to HIV and have not been on PrEP, PEP is the appropriate response. See the separate article on PEP for detail on what to do and where to go.
Availability in Singapore
PrEP has been available in Singapore since 2016 and is prescribed at private clinics as well as at specialist centres including the National Centre for Infectious Diseases. It is not currently subsidised by the government, meaning the cost is borne out of pocket. Generic formulations are available and substantially reduce the cost compared to branded options. Cost and availability should be discussed at consultation.
Whether PrEP is right for you depends on your individual risk profile. Speak to a doctor for a proper assessment – sourcing or starting PrEP without clinical oversight and monitoring is not safe practice.
- Lee JWE, Lim SW, Choy CY, Wong CS. Pre-exposure prophylaxis for prevention of HIV infection. Singapore Medical Journal. 2023;64(10):624–628. doi:10.4103/singaporemedj.SMJ-2022-018. PMID: 37861643. PMC10645013.
- McCormack S, Dunn DT, Desai M, Dolling DI, Gafos M, Gilson R, Sullivan AK, Clarke A, Reeves I, Schembri G, Mackie N, Bowman C, Lacey CJ, Apea V, Brady M, Fox J, Taylor S, Antonucci S, Khoo SH, Rooney J, Nardone A, Fisher M, McOwan A, Johnson AM, Phillips AN, Gazzard B, Gill ON. Pre-exposure prophylaxis to prevent the acquisition of HIV-1 infection (PROUD): effectiveness results from the pilot phase of a pragmatic open-label randomised trial. Lancet. 2016;387(10013):53–60. doi:10.1016/S0140-6736(15)00056-2. PMID: 26364263. PMC4700047.
- Molina JM, Capitant C, Spire B, Pialoux G, Cotte L, Charreau I, Tremblay C, Le Gall JM, Cua E, Pasquet A, Raffi F, Pintado C, Chidiac C, Chas J, Charbonneau P, Delaugerre C, Suzan-Monti M, Loze B, Fonsart J, Peytavin G, Cheret A, Timsit J, Garès-Bernabe G, Geffroy M, Carette D, Rooney J, Terraube P, Bernaud C, Lorente N, Bourber P, Costagliola D, Meyer L, Glidden DV, Anderson PL, Buchbinder SP, Grant RM, Poizot-Martin I, Goujard C, Yazdanpanah Y, Delfraissy JF, Fagard C, Whilst D; ANRS IPERGAY Study Group. On-demand preexposure prophylaxis in men at high risk for HIV-1 infection. New England Journal of Medicine. 2015;373(23):2237–2246. doi:10.1056/NEJMoa1506273. PMID: 26624850.