On-demand (or intermittent) PrEP means taking pills only on days before and after having sex. It is sometimes referred to as 2-1-1 dosing. The Canadian PrEP guideline states that on-demand PrEP can be considered as an alternative form of PrEP for gbMSM only. This could be a good option for men who know in advance when they will have sex. There is no evidence to support the use of on-demand PrEP by other populations.
This involves taking:
- 2 pills between 2 and 24 hours before sex
- 1 pill 24 hours after the first dose
- 1 pill another 24 hours later
If a person continues to have sex, they should continue to take a pill every 24 hours until two days after the last time they have sex.
On-demand PrEP has only been studied using TDF + FTC. Evidence from several studies shows that it reduces the risk of sexual HIV transmission among gbMSM who take it consistently and correctly. No studies have been conducted to evaluate the effectiveness of this particular on-demand regimen in other populations.
An RCT (Randomized Control Trial) called IPERGAY evaluated the 2-1-1 on-demand method and found an 86% reduced risk of HIV infection among gbMSM in the on-demand PrEP group compared with those in a placebo group (two participants in the PrEP group got HIV but neither had PrEP drugs detected in their blood, demonstrating that they were not actually taking the study drug). Men in the RCT took four pills a week on average, implying that they were having sex about once a week. When the RCT was completed, IPERGAY continued as an open-label extension with all participants offered on-demand PrEP. Results from the open-label phase showed that one more HIV transmission occurred among 362 participants, over 515 person-years of follow-up (equivalent to following 515 people for one year). None of the three participants who got HIV over the entire course of the study had PrEP drugs detected in their blood, which means they were not adherent.