There are two different ways to take PrEP that have been studied and proven effective:
Both protocols are effective, but the research produced different conclusions about how to start and stop PrEP. The guidelines below are based on the (much larger) trials of daily PrEP and have been endorsed by the U.S. Centers for Disease Control and Prevention (CDC) and other global health authorities.
The following guidelines apply to daily PrEP. For more information about PrEP 2-1-1, see What Is PrEP 2-1-1?
The World Health Organization (WHO) PrEP guidelines state that "PrEP users should be advised that PrEP reaches protection after 7 [daily] doses. Pharmacological studies suggest that full protection may require 4 doses for anal sex and 7 doses for vaginal sex." Guidance from the United States CDC is, as usual, more conservative and states: "PrEP reaches maximum protection from HIV for receptive anal sex at about 7 days of daily use. For receptive vaginal sex and injection drug use, PrEP reaches maximum protection at about 21 days of daily use." Both the WHO and CDC estimates indicate how long it take for PrEP to reach maximum protective levels There is evidence from this small research study (summarized in this article) that that very good protection is achieved after only one or two doses.
Some health authorities have recommended that you continue to take PrEP daily for 28 days after your last possible exposure to HIV, but the evidence for this recommendation was never strong, and most authorities now agree this is unnecessary. The research on PrEP 2-1-1 suggests that continuing Truvada for only a few days after your last possible exposure to HIV is sufficient. There is no clear consensus, but the July 2019 recommendation from WHO states that "PrEP programmes for men who have sex with men that offer both the event-driven and daily approaches usually advise that PrEP can be stopped after two daily doses following the last sexual exposure."
IMPORTANT! If you have an active hepatitis B infection, talk to your doctor before you stop PrEP. Stopping Truvada or Descovy can cause active hepatitis B to get worse quickly.
We do not have good evidence about how well these guidelines may apply to you. The guidelines came from studies of daily PrEP that were limited to cisgender men who have sex with men and cisgender and transgender women.
No clinical studies have specifically investigated Truvada or Descovy in injection drug users. However, one study found a nearly 50% reduction in HIV incidence among injection drug users who took daily TDF, which is one of the two drugs in Truvada and is similar to one of the drugs in Descovy, and so it is almost certain that Truvada provides similar protection against HIV. As a result, CDC guidance states that PrEP highly effective for preventing HIV in people who inject drugs. There is no data on how long you need to take daily Truvada in order to have protection against an injection exposure to HIV. However, studies have shown that it takes 21 days of daily dosing for Truvada to reach maximum concentration in blood. There is no medical reason to think it would take longer than that for Truvada to provide maximum protection from an injection exposure to HIV.