PrEP 2-1-1, precisely
On-demand dosing is as effective as daily PrEP when it is timed correctly. Most of the confusion is about what happens when the plan changes.
Event-driven PrEP means taking the drug around sex rather than every day. It is called 2-1-1 for the shape of the schedule: two pills before, one at 24 hours, one at 48. Done properly it is not a weaker option — the trial that established it found the same level of protection as daily dosing.
The catch is that "properly" is doing real work in that sentence, and the schedule is easier to describe than to follow at four in the morning.
Read the timing off the first dose, not off the sex. This is the single most common mistake. The two follow-up pills are at 24 and 48 hours after you took the double dose, whatever happened in between.
Keep taking one pill every 24 hours for as long as it continues, then two more daily doses after the last encounter. A long weekend does not mean repeating the double dose — you load once and stay on the daily rhythm until two days after you finish.
Who it is validated for
This matters more than the schedule, because 2-1-1 is not a universal alternative to daily dosing.
- Cisgender men having anal sex. This is the population the evidence covers, and where the regimen is recommended.
- TDF/FTC only — the older generic combination. TAF/FTC (Descovy) has not been validated for on-demand use and should not be substituted into this schedule.
- Not for vaginal sex. Drug levels in vaginal tissue rise more slowly, and the 2-hour lead time is not enough.
- Not if you have hepatitis B. Starting and stopping this drug repeatedly can cause a serious flare. Daily dosing, with monitoring, is the option there.
When the plan changes
| What happened | What to do |
|---|---|
| Took the double dose, then nothing happened | Take the two follow-up pills anyway, at 24 and 48 hours. Finishing the course is how the regimen is designed; stopping early is the version with no evidence behind it. |
| Sex happened sooner than expected — under 2 hours after the double dose | Levels may not be there yet. Not a disaster, but treat it as a potential exposure and consider the 72-hour questions. |
| Forgot the double dose entirely; sex already happened | 2-1-1 does not work retroactively. This is a PEP conversation, not a PrEP one, and it is time-limited. |
| Missed the 24-hour pill; remembered at 30 hours | Take it as soon as you remember and keep the 48-hour dose on the original clock, not shifted. |
| Doing this most weekends | Worth pricing out daily dosing. At four or more sessions a month the pill count converges, and daily is more forgiving of a chaotic schedule. |
What it does not cover
HIV, and nothing else. Syphilis, chlamydia and gonorrhoea are entirely unaffected, and the three-monthly screening that comes with a PrEP prescription is doing as much work for your health as the drug is. Do not skip the appointments because you feel fine — most of what it catches has no symptoms.
Questions people actually ask
Two hours before feels tight. Can I take it earlier?
Yes — the window is 2 to 24 hours, and the far end of it is much more comfortable. Taking the double dose in the early evening for a night out is completely standard and removes the scramble entirely.
Does it need to be with food?
Absorption is better with food, and the nausea people report from the double dose is noticeably less on a full stomach. Not a rule, but a good habit.
I am on 2-1-1 and had sex three days in a row. What was the schedule?
Two pills before the first encounter, then one every 24 hours through Tuesday if the last sex was Sunday — one pill a day during, plus two days after the last one. Six pills total in that example, not three separate 2-1-1 courses.
Can I switch between daily and on-demand?
Yes, and plenty of people do it seasonally. Coming off daily onto 2-1-1 needs no loading dose if you have been taking it consistently; going the other way, start daily and you are covered for anal sex after about a week. Worth confirming with whoever prescribes it.
- Molina JM et al. On-demand preexposure prophylaxis in men at high risk for HIV-1 infection. ANRS IPERGAY. N Engl J Med 2015;373:2237–2246.
- CDC — Sexually Transmitted Infections Treatment Guidelines, 2021 (PDF)
- CDC — Clinical guidance for STIs
This page is general information about how prevention and testing work, not medical advice, and it cannot tell you what is right for your situation. Guidance differs between countries and changes over time. Anything specific to you is a conversation for a clinician or sexual health service. Candor is a personal record-keeping app; it does not diagnose, treat or prescribe.
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The dosing schedule, tracked
Candor handles 2-1-1 against a specific encounter — the loading dose, the two follow-ups, and the extra days when sex continues.
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