Last updated: July 2026
On-Demand PrEP (2-1-1 Dosing): PrEP Only When You Need It
Daily pills don't make sense for everyone. If sex is something you plan rather than something that just happens, on-demand "2-1-1" PrEP — pills taken around the encounter only — offers near-daily protection with a fraction of the medication. Here's how it works and its very important fine print.
The 2-1-1 schedule
- 2 pills of TDF-FTC (Truvada/generic) taken 2–24 hours before sex (closer to 24 is better),
- 1 pill 24 hours after the double dose,
- 1 pill 24 hours after that.
- If sex continues across multiple days: keep taking one daily pill until 48 hours after the last encounter.
The evidence
The IPERGAY trial (France/Canada) tested exactly this schedule in men who have sex with men and found an 86% reduction in HIV acquisition versus placebo — and in follow-up open-label use, effectiveness approached that of daily dosing among consistent users. The IAS and WHO endorse 2-1-1 for MSM; the CDC acknowledges it as an off-label option men and their clinicians may choose. "Off-label" here means the FDA label doesn't include it, not that it's fringe — plenty of prescribers, including our reviewing team, support well-selected patients using it.
Who 2-1-1 is for — and emphatically not for
For: cisgender men who have sex with men, whose encounters are plannable at least 2 hours ahead, comfortable with a slightly more complex schedule.
Not for: anyone at risk through vaginal sex (tissue drug levels build too slowly — daily dosing is required for protection), people with hepatitis B (intermittent TDF can trigger dangerous flares — this is a hard stop that requires a hepatitis B test before choosing 2-1-1), and people whose sex is spontaneous enough that the lead-time requirement fails in practice. Descovy has no on-demand evidence; 2-1-1 is a TDF-FTC strategy only.
Practical realities
Most 2-1-1 users keep a standard 30-pill bottle that simply lasts months. You still need quarterly HIV/STI testing like any PrEP user. The most common real-world failure is the "before" dose — sex arriving sooner than planned. Many users solve this by switching flexibly: daily dosing during high-activity periods (a trip, a new relationship), 2-1-1 during quiet ones. That flexibility is legitimate and worth designing with your prescriber.
Bottom line
2-1-1 is a well-evidenced, guideline-supported option for the right person — roughly: MSM, hepatitis-B negative, plannable encounters. If that's you, raise it at a telehealth consult; if the fit is imperfect, every-two-months injections solve the same "I don't want a daily pill" problem with less fine print.