The 2-1-1 PrEP schedule is a way to take HIV prevention medication only around the times you have sex, rather than every day. You take 2 pills 2 to 24 hours before sex, 1 pill 24 hours after that first dose, and 1 pill 24 hours after the second dose. It’s sometimes called “on-demand” or “event-driven” PrEP, and in clinical trials it reduced HIV risk by up to 97%.
The Exact Dosing Timeline
The schedule works in three steps, all timed around your first double dose:
- Before sex: Take 2 pills between 2 and 24 hours before sexual activity. This is your loading dose. The closer to 2 hours beforehand you can manage, the tighter your window of protection, but any point in that 2-to-24-hour range works.
- First follow-up: Take 1 pill 24 hours after the first double dose.
- Second follow-up: Take 1 pill 24 hours after the second dose (so 48 hours after the original double dose).
If you have sex again on consecutive days, you continue taking 1 pill every 24 hours until your last sexual encounter, then take two more daily single pills after that final encounter. The key principle is: the two follow-up doses always come after the last time you had sex.
A Practical Example
Say you plan to have sex Saturday evening. You’d take 2 pills Saturday afternoon, at least 2 hours before. Sunday afternoon, you take 1 pill. Monday afternoon, you take your final pill. That’s it: four pills total for a single event.
Now imagine you also have sex Sunday night. You’d still take the Sunday pill as planned, then continue with 1 pill Monday and 1 pill Tuesday, since those two follow-up doses now trail your most recent encounter.
Which Medication and Who It’s For
The 2-1-1 method uses a combination pill containing tenofovir disoproxil fumarate and emtricitabine, commonly known by the brand name Truvada (or its generics). The other PrEP pill, Descovy, has not been studied for on-demand use.
This schedule is currently supported by evidence only for gay and bisexual men and transgender women who have sex with men. It is not recommended for people having vaginal or frontal sex. The reason is biological: vaginal and cervical tissue need far more exposure to reach protective drug levels. Vaginal tissue requires roughly 20 daily doses to achieve maximum protection, so a few pills around a single event aren’t enough.
The CDC notes that 2-1-1 dosing is not FDA-approved and is not officially recommended in their guidelines, but it can be prescribed off-label to adult gay and bisexual men who meet certain criteria: they request non-daily dosing, they have sex infrequently (less than about once a week), and they can anticipate sex or delay it long enough to get that 2-hour head start. It’s also not appropriate for people with active hepatitis B, since intermittent use of this medication can trigger liver flares.
How Well It Works
The main evidence comes from the IPERGAY trial conducted in France and Canada. In the randomized phase, on-demand PrEP reduced HIV incidence by 86% compared to placebo. When the trial moved to an open-label phase where all participants knew they were getting the real medication, HIV incidence dropped to 0.19 per 100 person-years, compared to 6.60 in the original placebo group. That translates to a 97% reduction.
These numbers are comparable to daily PrEP when people actually follow the dosing schedule. The protection depends entirely on taking all the doses correctly, especially the loading dose before sex and both follow-up pills afterward.
What Happens If You Miss a Dose
If you miss one of your follow-up pills, take 2 pills as soon as you realize the mistake and contact your prescribing clinic. A missed dose means your drug levels may have dropped below protective thresholds. If you missed the loading dose entirely and had unprotected sex, you’ll want to talk to a provider about post-exposure prophylaxis (PEP), which is a different medication regimen started within 72 hours of potential exposure.
Ongoing Monitoring
Even though you’re not taking pills daily, you still need regular check-ins with a healthcare provider. This typically includes HIV testing (since PrEP should never be taken by someone who already has HIV), screening for other sexually transmitted infections, and periodic kidney function checks. The IPERGAY trial enrolled participants who had normal kidney function at baseline, and monitoring ensures the medication isn’t causing problems over time.
Is 2-1-1 Right for You?
The 2-1-1 method works best for people whose sexual activity is occasional and somewhat predictable. If you’re having sex multiple times a week, daily PrEP is simpler and removes the need to plan doses around each encounter. If you have sex a few times a month and can reliably take pills on schedule, on-demand dosing means fewer pills, potentially fewer side effects, and lower cost.
The schedule does require some planning. You need at least a 2-hour window before sex, and you need to commit to those two follow-up pills even after the encounter is over. People who might struggle with a multi-step regimen, including adolescents or those dealing with active substance use disorders, are generally better served by daily dosing according to CDC guidance.