What is the evidence for i-Base info about stopping PrEP?
In terms of stopping Prep, i-Base advises to wait 7 days with daily dosing after your last sexual encounter. However, most sites recommend 28 days after the last sexual encounter (same as taking PEP). Is there any particular research that the 7-day suggestion is based upon? Thanks very much.
Thanks for your question – it is always good to go further to look at the evidence and then in details at those studies.
I think your question refers to information in the i-Base UK guide to PrEP. 
In this case, the recommendation is based on the UK (BHIVA/BASHH) guidelines. 
The evidence to support the safety of stooping after seven days varied depending on how PrEP was being used.
If this was for anal sex, then the IPERGAY study in gay men proved this was safe because it was testing on-demand dosing that ended 48 hours after the risk. As there were no infections with this dosing, daily dosing for seven days is also proved to be safe. 
The protection during vaginal sex is different because PrEP meds are absorbed less well in these tissues, and drop more quickly after dosing.
Two studies were reference for the conclusion that seven days would be enough though when daily dosing was being used by women and trans people for vaginal protection. [4. 5]
Hyperlinks to these references are also included below.
As a comment, many aspect of PrEP, including dosing, have come from interpreting results from different studies. For example, there is no direct evidence for the target levels of PrEP drugs, or the thresholds for efficacy, or even in what types of tissue or cells these levels should be measured.
We do know PrEP is close to 100% effective though when taken as prescribed.
Some guidelines might have been over cautious in their initial decision to recommend 28 days. Similarly, some guidelines still don’t recommend on-demand dosing.
- i-Base. How to stop PrEP safely. UK guide to PrEP.
- BHIVA/BASHH guidelines on the use of HIV pre-exposure prophylaxis (PrEP), 2018
- Molina JM, Capitant C, Spire B et al. On-demand preexposure prophylaxis in men at high risk for HIV-1 infection. N Engl J Med 2015; 373: 2237–2246.
- Seifert SM, Glidden DV, Meditz AL et al. Dose response for starting and stopping HIV preexposure prophylaxis for men who have sex with men. Clin Infect Dis 2015; 60: 804–810.
- Cottrell ML, Yang KH, Prince HM et al. A translational pharmacology approach to predicting outcomes of preexposure prophylaxis against hiv in men and women using tenofovir disoproxil fumarate with or without emtricitabine. J Infect Dis 2016; 214: 55–64.