The landmark iPrEx study demonstrated the feasibility of taking Truvada for pre-exposure prophylaxis (PrEP). It also demonstrated that success depends on adherence. The trial participants receiving Truvada had a 44% reduced risk of becoming infected overall, but those who were actually taking the medication had a 92% risk reduction.
The iPrEx team have now turned to the question, how often one has to take the pill to remain protected. Testing at 2, 4, and 7 pills a week, they found the minimum number of pills per week is 4.