Key takeaways
- On-demand PrEP means taking 2 pills 2 to 24 hours before sex, then 1 pill 24 hours later and 1 more pill 24 hours after that.
- Taken correctly, it gave very high protection against HIV in studies of gay, bisexual and other men who have sex with men.
- WHO recommends it only for men who have sex with men; it is not advised for vaginal sex under WHO guidance.
- People with hepatitis B usually need daily PrEP instead, after a doctor’s assessment.
- You need an HIV test before starting and regular tests while you use it.
What is PrEP 2-1-1?
PrEP 2-1-1 is a way of taking HIV prevention pills only around the times you have sex, instead of every day. You take a double dose before sex and one pill on each of the next two days, which is where the name comes from.
It is also called on-demand or event-driven PrEP. It uses the same tablets as daily PrEP, so the choice is about the schedule, not the medicine. For the basics of PrEP itself, see how PrEP works at our Bangkok clinic.
The 2-1-1 dosing schedule, step by step
The schedule is short, but the timing of each dose matters. According to the WHO technical brief on 2+1+1 event-driven PrEP, it works like this:
- 2 pills between 2 and 24 hours before sex.
- 1 pill 24 hours after the double dose.
- 1 pill 48 hours after the double dose.
An example on-demand PrEP schedule
| Dose | When | Example |
|---|---|---|
| Double dose (2 pills) | 2–24 hours before sex | Saturday 18:00, sex later that night |
| Second dose (1 pill) | 24 hours after the double dose | Sunday 18:00 |
| Third dose (1 pill) | 48 hours after the double dose | Monday 18:00 |
Take the two follow-up pills at about the same time of day as the double dose.
If you have sex on several days in a row
Keep taking one pill every 24 hours for as long as you are having sex. Then take one pill a day for two more days after the last time you had sex, and stop.
Starting again after a break
Guidelines differ slightly on restarting when your last pill was less than a week ago. Many services now advise a double dose every time you restart, so ask your doctor which rule to follow and write it down.
How effective is on-demand PrEP?
In the IPERGAY trial of men who have sex with men, 2-1-1 dosing reduced the risk of HIV by 86% compared with a placebo. The few infections in the PrEP group happened in people who had stopped taking it.
In the open-label follow-up of that study, the reduction was 97%. In the large Prevenir study in France, the only infections among on-demand users were again in people who had stopped PrEP, which suggests that correct use gives very high protection.
PrEP 2-1-1 effectiveness vs daily PrEP
| Question | On-demand (2-1-1) | Daily PrEP |
|---|---|---|
| Who it is for | Men who have sex with men | Anyone at risk of HIV |
| Protection when used correctly | Very high for anal sex | Very high for all types of sex |
| Pills needed | Only around sex | One every day |
| Planning needed | At least 2 hours before sex | None once protected |
| Best suited to | Sex less than about twice a week | Frequent or unplanned sex |
| Hepatitis B | Usually not advised | Possible with monitoring |
Protection depends on taking every dose on time. PrEP does not protect against other STIs, so regular STI testing in Bangkok is still part of using it.
Who is on-demand PrEP for?
The evidence for 2-1-1 comes mainly from studies of gay, bisexual and other men who have sex with men. WHO suggests it may suit men who have sex with men who:
- have sex less often, for example fewer than about two times a week on average
- can plan sex at least 2 hours ahead, or delay it until 2 hours after the double dose
- find daily pills harder to keep up, or prefer to take fewer pills
You can switch between daily and on-demand PrEP as your life changes. Our guide to the differences between PEP and PrEP is a useful starting point.
Who should not use the 2-1-1 schedule?
WHO’s 2019 guidance says on-demand PrEP is not appropriate for:
- cisgender women and transgender women
- transgender men having vaginal or frontal sex
- men having vaginal or anal sex with women
- people with chronic hepatitis B
The main reason is that drug levels build more slowly in vaginal tissue, and there are few studies outside men who have sex with men. WHO also advises caution for transgender women taking feminising hormones, which may lower drug levels.
Some newer national guidance goes further. The 2025 BASHH/BHIVA UK PrEP guidelines describe event-based options for other types of sex, including a longer 7-day course after receptive vaginal sex, but based on weaker evidence. Your doctor will follow the guidance that applies to you.
Why hepatitis B matters
PrEP medicines also act on hepatitis B, and stopping and starting them can trigger a liver flare in people who have it. This is why you are tested first, and why daily PrEP is generally preferred if you have hepatitis B.
PrEP 2-1-1 missed dose: what to do
Late doses happen, and guidelines give clear rules. The UK guidance says:
- Late first dose after sex: if you took at least one pill before sex, you can still take it up to 48 hours after sex, then the last pill 24 hours later.
- More than 48 hours late: take a pill, keep taking one a day, and contact a clinic for advice.
- No pills before sex at all: start with a double dose as soon as possible, ideally within 24 hours, keep taking one daily, and ask a clinic about PEP.
If you had sex without any PrEP in your system, PEP within 72 hours may be the right step. Read what to do after a possible HIV exposure and our guide to how PEP works.
Tests before and during on-demand PrEP
You need a reliable HIV test before starting, because taking PrEP with an undiagnosed infection can lead to drug resistance. A home kit alone is not enough; our article on HIV self-tests in Thailand explains why.
WHO advises an HIV test every 3 months for both daily and on-demand users, even if you only use PrEP now and then. Your doctor will also check your kidneys, test for hepatitis B and offer STI screening; see HIV testing at our clinic.
Side effects of on-demand PrEP
Most people have mild or no side effects. The UK guidelines note that taking the double dose with food may reduce stomach upset, and the two tablets can be taken 6 to 12 hours apart as long as the second is at least 2 hours before sex.
For what to expect and when to call, see our guide to PrEP side effects in the first weeks.
Getting on-demand PrEP in Bangkok
At HIV PEP Bangkok, near BTS Asok and MRT Sukhumvit, a PrEP visit includes screening, a kidney check and a 30-day supply. English- and Thai-speaking doctors talk you through daily and on-demand options in a private room.
If you are visiting Thailand, our sexual health guide for travellers in Bangkok covers carrying PrEP and getting check-ups while you are here.
Frequently asked questions about on-demand PrEP
Can I switch from daily PrEP to 2-1-1?
Yes, if on-demand dosing is suitable for you. To stop daily PrEP safely, keep taking it for two days after the last time you had sex, then follow the 2-1-1 schedule next time.
Does on-demand PrEP work for vaginal sex?
WHO does not recommend 2-1-1 for vaginal sex, because drug levels build more slowly in vaginal tissue. Daily PrEP is the usual choice.
Do I need HIV tests if I only use PrEP occasionally?
Yes. An HIV test every 3 months is advised for on-demand users too, because an infection during a break could go unnoticed.
Can I use 2-1-1 with any PrEP tablet?
The schedule was studied with tenofovir disoproxil plus emtricitabine. Evidence for other PrEP tablets taken this way is more limited, so use the tablets your doctor prescribes for on-demand dosing.
If you would like to talk through whether 2-1-1 suits you, contact our Bangkok clinic by LINE, WhatsApp or phone to arrange a PrEP visit.
