What Is On-Demand PrEP (2-1-1)? Who Is It For, and How Do You Take It?
Besides taking PrEP daily, there’s another approach called On-Demand PrEP — taking it only around an expected risk, using the 2-1-1 schedule. It’s an option for some people who aren’t at frequent risk and can plan ahead.
This article explains what On-Demand PrEP is, how the 2-1-1 schedule works, who it is and isn’t for, its cautions, and how it differs from daily PrEP. It draws on WHO, U.S. CDC, and Thai Department of Disease Control guidance. If PrEP is new to you, first read PrEP for HIV and On-Demand PrEP.
What is On-Demand PrEP?
On-Demand PrEP (also called event-driven PrEP) means taking PrEP only around an expected risk, rather than every day, using a schedule called 2-1-1. It has research support among men who have sex with men (MSM) for anal sex, and is endorsed by WHO for this group. Importantly, it requires planning ahead, since you must start before the risk. See the prevention overview at HIV prevention.
How does the 2-1-1 schedule work?
| Timing | Pills | Detail |
|---|---|---|
| Before sex | 2 pills | Take 2 pills 2–24 hours before |
| 24 hours after first dose | 1 pill | Take 1 pill |
| 24 hours after second dose | 1 pill | Take 1 more pill |
- 2 pills — taken 2–24 hours before sex.
- 1 pill — 24 hours after the first dose.
- 1 pill — 24 hours after the second dose.
Hence the name “2-1-1.” Taking the full schedule on time matters for effectiveness. If sex continues over several days, a doctor may advise continuing one pill daily as instructed — always follow your doctor’s directions.
Who is On-Demand for?
Groups with research support (WHO-endorsed):
- Men who have sex with men (MSM)
- For anal sex
- Those who can plan ahead and aren’t at very frequent risk
Groups without enough data should use daily PrEP instead:
- Vaginal sex
- People who share injecting equipment
- Transgender people (limited data)
For groups without data supporting 2-1-1, daily PrEP is more appropriate as it has broader support. Always consult a doctor to choose what suits you. Read more on daily PrEP.
Daily or On-Demand — which to choose?
- Daily — taken every day, most research support, suits everyone at risk, especially those at frequent or unpredictable risk.
- On-Demand — taken only around a risk via 2-1-1, supported only for MSM, requires planning ahead, suits those at infrequent, predictable risk.
In short, if your risk is frequent or unpredictable, daily is usually better; if it’s infrequent and you can plan ahead and are in a supported group, On-Demand is a good option. A doctor helps assess which suits you.
Cautions with On-Demand
- Must plan ahead — you must start 2–24 hours before a risk; missing that window lowers effectiveness.
- Supported only for some groups — those without data should use daily.
- Take the full schedule on time — incomplete or mistimed doses affect protection.
- Some health conditions may not suit it — some conditions require checking with a doctor about suitability.
So On-Demand isn’t for everyone and must be started only under a doctor’s guidance — don’t decide to use it on your own.
Benefits of On-Demand
For those in a supported group who suit this approach, On-Demand has several benefits. It means no daily pill — suited to those at infrequent risk, so no continuous daily dosing; less medication in some cases — for occasional risk, it may use fewer pills than daily; flexibility — you can plan protection around expected risk periods; and it remains effective in supported groups — when taken correctly and on time. Still, these benefits only apply if you’re in a suitable group and follow the schedule strictly. For frequent or unpredictable risk, daily may be more convenient and appropriate. Weighing the pros and cons with a doctor helps you choose what’s best for you.
Consult a doctor and get monitoring
- HIV test before starting — confirm you don’t already have HIV.
- Suitability assessment — a doctor assesses whether On-Demand suits you.
- Regular monitoring — see a doctor and get follow-up as scheduled.
- Guidance on correct dosing — for maximum protection.
If interested in On-Demand, start by consulting a doctor or clinic that provides it. See HIV testing at HIV testing.
On-Demand protects against HIV only
Like all PrEP, On-Demand protects against HIV only — not other STIs like syphilis or gonorrhoea. So use it with condoms for broader protection and get regular sexual-health check-ups. Understanding this limit helps you plan complete sexual-health care.
What if you miss a dose in the schedule?
- Don’t guess or adjust the schedule yourself; ask a doctor or clinic for advice.
- Missing a key dose may leave protection incomplete; use other protection too, such as condoms.
- If you had a high risk and are unsure about protection, see a doctor promptly.
Following your doctor’s directions closely helps On-Demand work as intended and reduces confusion when the unexpected happens.
Conclusion
On-Demand PrEP means taking HIV-prevention pills only around an expected risk, via 2-1-1: 2 pills 2–24 hours before, then 1 pill 24 hours after the first dose, and 1 more 24 hours after the second. It’s supported only for men who have sex with men for anal sex, and WHO-endorsed for this group. Other groups — vaginal sex, people who share injecting equipment, and transgender people — should use daily PrEP, which has broader support. On-Demand requires planning ahead, taking the full schedule on time, and starting under a doctor’s care with an HIV test before and regular monitoring; and it protects against HIV only, so use it with condoms. If interested, consult a doctor or clinic. See more at On-Demand PrEP and PrEP for HIV.
