This PrEP on-demand calculator turns one date and time into a full 2-1-1 schedule, so you know exactly when to take each pill before and after sex.
Enter when you expect sex to happen and the planner works backwards and forwards for you. It shows the double starting dose and the two single follow-up doses, and reminds you what to do if sex carries on over several days. It is a planning aid, not a prescription, so use it alongside advice from your doctor.
Think of it as a start PrEP calculator for a single event: you give it one moment, and it builds the rest of the plan around it.
How to use the planner
The PrEP on-demand calculator needs only a little information. You don’t need to create an account, and nothing you type is a medical record.
- Choose the date and the earliest time you think sex could happen.
- Choose how many hours before sex you’ll take the double dose. Anything from 2 to 24 hours works; never less than 2 hours.
- Read the schedule it produces: the double dose, then one pill 24 hours and one pill 48 hours after it.
- Tap “Add reminders to my calendar” to save all three pill times, each with an alert.
If your plans move, simply run the PrEP on-demand calculator again with the new time. Plans changing is normal; the rules stay the same.
The 2-1-1 rules in plain words
On-demand dosing uses the same tablet as daily PrEP: tenofovir disoproxil with emtricitabine (TDF/FTC), sold as Truvada or as generics such as Teno-EM, Tenof-EM and Ricovir-EM. If you’re comparing packs, our guide to generic PrEP brands in Thailand explains which ones qualify.
Below is the full PrEP 2-1-1 dosing schedule the planner follows. With on-demand PrEP, when to take each pill depends only on the time of your loading dose, not on the time you actually have sex.
Each dose and when to take it
| Dose | How many pills | When |
|---|---|---|
| Loading dose | 2 | 2–24 hours before sex |
| Second dose | 1 | 24 hours after the loading dose |
| Third dose | 1 | 48 hours after the loading dose |
| If sex continues | 1 a day | Every 24 hours until 2 days after the last sex |
Keep the doses at roughly the same clock time as the loading dose. CATIE’s plain-language explainer on how PrEP 2-1-1 works sets out the same schedule if you’d like a second source.
Getting the on-demand PrEP timing right matters most at the start. The two-pill dose needs time to reach protective levels, which is why the 2-hour minimum is not negotiable.
Who can use on-demand dosing
On-demand dosing has only been studied for anal sex, so it suits a narrower group than daily PrEP.
It’s an option for:
- cisgender men who have sex with men;
- trans women who are not taking estradiol-based hormones.
It is not suitable for:
- vaginal or front-hole sex, because the medicine takes longer to build up in those tissues;
- anyone with hepatitis B, since starting and stopping tenofovir repeatedly can cause the liver infection to flare;
- trans women taking estradiol, or anyone exposed through injecting drugs.
If one of these applies to you, daily PrEP is the safer choice. Not sure which fits? Try the one-minute PrEP self-check or read who PrEP is designed for. Before your first dose you’ll also need an HIV test, a kidney check and a hepatitis B test, all covered in our guide to PrEP tests and results.
Worked examples with real clock times
These three patterns cover most situations people plan for, and each follows the same on-demand PrEP schedule. Times are examples; your own schedule follows your loading dose.
Three common plans, step by step
| Situation | Loading dose (2 pills) | Single doses |
|---|---|---|
| One night out, sex Saturday 23:00 | Saturday 20:00 | Sunday 20:00, Monday 20:00 |
| Long weekend, sex Friday to Sunday | Friday 10:00 | Daily at 10:00 Saturday to Tuesday |
| Short notice, sex in 3 hours | Now (at least 2 hours before) | Same clock time on the next two days |
In the long-weekend example, the last sex is on Sunday, so you keep taking one pill a day until Tuesday, two days later. That is why the weekend uses four single doses instead of two.
Travelling across time zones? Stick to 24-hour gaps from your loading dose rather than local mealtimes. Our page on PrEP for visitors to Thailand has more tips for trips.
Late, missed or early doses
Small slips happen. The general guidance is to take a late single dose as soon as you remember, as long as it’s within a few hours of the planned time, and then carry on at the original times. Never take two single doses together to catch up.
Missing the second or third pill altogether, or having sex less than 2 hours after the loading dose, can lower protection. Our detailed guide to what to do after a missed PrEP dose works through each case hour by hour.
If you last took on-demand PrEP within the past 7 days, restart with one pill; if it’s been longer, begin again with the full double dose.
Exposed in the last 72 hours without PrEP protection? You need PEP, not PrEP, and it works best the sooner it starts. Ask us about urgent PEP now.
Set calendar reminders that actually work
The schedule only protects you if the follow-up doses happen, and the 48-hour pill is the one people forget most often. A few habits help:
- Save the three pill times straight from the planner; each calendar entry comes with an alert 10 minutes before.
- Name them clearly, for example “pill 2 of 4”, so the reminder still makes sense when you’re tired.
- Keep a spare pill in your wallet or bag in a small sealed case, with the original box at home.
- If friends or a partner use on-demand PrEP too, a shared reminder can help everyone.
When you’re ready to begin, you can book a PrEP consultation in Bangkok. Our doctors speak English, and you can message us on LINE or WhatsApp first. If you later want to switch to daily dosing, read how to start and stop PrEP safely.
Questions people ask
How long before sex should I take on-demand PrEP?
Take the two-pill loading dose between 2 and 24 hours before sex. Less than 2 hours is too late for the first dose to protect you.
When do I take PrEP 2-1-1 if sex happens over several days?
Take one pill every 24 hours for as long as sex continues, then one pill a day for the two days after the last time. The planner shows the first three doses; add one pill a day for each extra day of sex.
Can I use the 2-1-1 schedule for vaginal sex?
No. On-demand dosing hasn’t been shown to protect during vaginal or front-hole sex, so daily PrEP is recommended instead.
Is on-demand PrEP as effective as daily PrEP?
For cis men who have sex with men who follow the schedule exactly, studies show high protection, similar to daily use. WHO includes this event-driven approach in its guidance on PrEP options.
Does on-demand PrEP protect against other STIs?
No. PrEP only lowers the chance of getting HIV. Condoms and regular sexual health checks help with other infections.
Written by the HIV PrEP Bangkok team, based on WHO and CATIE guidance. Last updated 2 October 2026.