On-Demand PrEP


PrEP is highly effective for preventing HIV when taken as prescribed, but varies widely across US demographics.

  • PrEP reduces the risk of getting HIV from sex by about 99%!
    • PrEP reaches maximum protection from HIV for receptive anal sex at about 7 days of daily use.
    • PrEP reaches maximum protection from HIV for receptive vaginal sex at about 21 days of daily use.
    • No data are available for insertive anal sex or insertive vaginal sex.
  • There is less information about how effective PrEP is among people who inject drugs, but it appears to reduce the risk of getting HIV by about 74-84%.
    • PrEP reaches maximum protection from HIV for receptive vaginal sex and injection drug use at about 21 days of daily use.

There are 2 medications approved for pre-exposure prophylaxis (PrEP) for HIV: Truvada and Descovy.

  • Both Truvada and Descovy contain emtricitabine, but differ in their formulation of tenofovir. 


CDC recommends that PrEP be considered for people who are HIV negative and who have had anal or vaginal sex in the past 6 months and:

  • have a sexual partner with HIV (especially if the partner has an unknown or detectable viral load), or
  • have not consistently used a condom, or
  • have been diagnosed with a sexually transmitted disease (STD) in the past 6 months.

PrEP is also recommended for people without HIV who inject drugs and:

  • have an injection partner with HIV, or
  • share needles, syringes, or other equipment to inject drugs.

PrEP should also be considered for people without HIV who have been prescribed non-occupational post-exposure prophylaxis (PEP) and:

  • report continued risk behavior, or
  • have used multiple courses of post-exposure prophylaxis.

On-Demand PrEP

Taking PrEP only when you are at risk for getting HIV is known as “on-demand” PrEP (aka event-driven PrEP). On-demand PrEP has been studied in MSM on a “2-1-1” schedule, meaning you take 2 pills 2-24 hours before sex, 1 pill 24 hours after the first dose, and 1 pill 24 hours after the second dose. 

Source: WHO
  • Key point: On-demand PrEP has not been adequately studied in non-MSM populations, thus current recommendations only advocate its use in MSM.

Multiple health organizations around the world offer guidance for on-demand PrEP as an alternative to daily PrEP for MSM at risk for HIV, including the WHO:

Source: WHO

On-demand use is not currently part of CDC’s guidelines for PrEP use, which still recommends daily use for people at risk for HIV. Furthermore, taking PrEP once a day is currently the only FDA-approved schedule for taking PrEP to prevent HIV. 

Criticisms of on-demand PrEP include that some patients may find it confusing and may have difficulty remembering to take non-daily therapy. There is also a theoretical risk of HIV drug resistance in the setting of suboptimal adherence. 

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