On-demand PrEP, also called 2-1-1 or event-driven PrEP, means taking PrEP around the times you have sex instead of every day. You take two pills 2–24 hours before sex, one pill 24 hours after the first dose and one more 48 hours after the first dose.
It works well for some people having anal sex, but it isn't for everyone. It is not suitable for vaginal or front-hole sex, for people with hepatitis B or for people taking oestradiol.
If you're new to PrEP, our guide to what PrEP is and how it works is a good place to start.
How 2-1-1 PrEP works, step by step
The name tells you the pattern: 2 pills, then 1, then 1. The timing of each dose is counted from the first double dose, not from when you had sex.
- Two pills between 2 and 24 hours before sex.
- One pill 24 hours after the double dose.
- One pill 48 hours after the double dose.
The double dose is what builds protection quickly. If you take it less than 2 hours before sex, there may not be enough medicine in your body yet.
The 2-1-1 timeline
| Dose | When | Pills |
|---|---|---|
| First | 2–24 hours before sex | 2 |
| Second | 24 hours after the first dose | 1 |
| Third | 48 hours after the first dose | 1 |
| Extra (if sex continues) | Every 24 hours | 1 |
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 a day until 2 days after the last time you had sex.
For example, a long weekend with sex on three nights in a row uses six pills in total. The PrEP planner can work out the exact dose times for your own dates.
Who on-demand PrEP suits, and who should use daily
On-demand PrEP was studied mainly in gay and bisexual men and other people having anal sex. It suits people whose sex is occasional and can be planned at least a couple of hours ahead.
On-demand may suit you if
- You only have anal sex.
- You have sex less than about twice a week.
- You can usually plan sex at least 2 hours in advance.
- You are happy to follow exact dose times.
You should use daily PrEP if
- You have vaginal or front-hole sex.
- You have hepatitis B, because starting and stopping PrEP can cause a flare.
- You take oestradiol, including as part of gender-affirming care.
- You often find it hard to time doses.
A doctor will assess your situation and check your hepatitis B status before you start. Our page on the tests before starting PrEP explains why that check matters.
Daily PrEP vs on-demand: which is better?
Neither is "better" for everyone. Both work very well when taken correctly; the right one depends on your body, your sex life and your routine.
Daily vs on-demand comparison
| Point | Daily PrEP | On-demand 2-1-1 |
|---|---|---|
| Suitable for | Everyone | Anal sex only |
| Hepatitis B or oestradiol | Yes, with advice | Not suitable |
| Planning needed | None once protected | Dose 2–24 h before sex |
| Pills used | One every day | Fewer, if sex is occasional |
| Easy to miss a dose? | Forgiving of the odd miss | Every dose matters |
According to i-Base's guide to on-demand PrEP, on-demand dosing was highly effective in the French IPERGAY study, and both schedules worked equally well among the men taking part. The catch is that every dose has to happen on time.
For more on how quickly each schedule protects you, read how long PrEP takes to work.
Can I switch between daily and 2-1-1?
Yes, many people do. Some use daily PrEP during a busy holiday and 2-1-1 at home, but talk to your doctor before you switch so the change is done safely.
Getting the timing right
Most problems with 2-1-1 come from timing, not from the medicine. Here is a worked example.
Say you expect to have sex on Friday at 8 pm. You take two pills at 2 pm on Friday, one at 2 pm on Saturday and one at 2 pm on Sunday.
If you have sex again on Saturday night, you add a pill at 2 pm on Monday. That covers the 2 days after your last sex.
Common mistakes to avoid
- Taking the first dose less than 2 hours before sex.
- Taking one pill instead of two at the start.
- Forgetting the 24-hour or 48-hour dose after a good night out.
- Using 2-1-1 for vaginal or front-hole sex.
If you are late with a dose, see our guide on what to do after a missed PrEP dose. Drinking can make doses easier to forget, so our page on PrEP and alcohol is worth a read too.
What if sex doesn't happen?
If you took the double dose and plans fell through, you don't need to carry on with the other pills. Ask your doctor how to restart next time, especially if the next chance of sex is only a few days away.
What on-demand PrEP doesn't do
Like daily PrEP, the 2-1-1 schedule only lowers the risk of HIV. It doesn't protect against syphilis, gonorrhoea, chlamydia or other STIs, so condoms and regular STI checks still matter.
It also can't be started after the event. If you had sex without protection in the last 72 hours and didn't take the double dose beforehand, you may need PEP, not PrEP. Don't wait to see what happens; ask a doctor the same day.
On-demand PrEP in Thailand and Pattaya
On-demand PrEP is used in Thailand, and the Thai generics contain the same active ingredients as the PrEP used in the main 2-1-1 research. Our guide to PrEP brands available in Thailand explains the options.
Whether you live in Jomtien or are visiting for a week, our guide to getting PrEP in Pattaya explains how the first visit works. Visitors planning a trip can also read when to start PrEP before your Thailand trip.
Not sure whether daily or 2-1-1 fits you? Message us on LINE or WhatsApp and tell us a little about your plans; a doctor will help you choose.
Frequently asked questions
Can I take on-demand PrEP 1 hour before sex?
No. The double dose needs to be taken at least 2 hours before sex, and up to 24 hours before, to build enough protection.
Is on-demand PrEP as effective as daily PrEP?
For anal sex, research shows it works very well when every dose is taken on time. Daily PrEP is more forgiving if you sometimes forget a pill.
Does on-demand PrEP work for women?
It is not recommended for vaginal or front-hole sex. Anyone at risk through vaginal or front-hole sex should use daily PrEP.
Can I use Descovy for 2-1-1?
Descovy has not been studied enough for 2-1-1, so it is not recommended. On-demand PrEP uses tenofovir disoproxil with emtricitabine, the medicine in Truvada and Thai generics.
Do I still need check-ups on 2-1-1?
Yes. Everyone on PrEP should have an HIV test every 3 months, plus other checks as your doctor advises.