Pre-exposure prophylaxis (PrEP) is a novel HIV-prevention strategy that would use antiretrovirals (ARVs) to protect HIV-negative people from HIV infection. In this strategy, people would take the medications before they were exposed to HIV, in the hope that it would lower their risk of infection. A large study, IPrEx, suggested that PrEP might be effective, but only if adherence were good – and in the study trial data, it was clear that this was not the case for many participants, who although they said that their adherence was good, were shown by blood tests to be much less adherent than claimed. A new US study has similarly found that although participants reported adherence levels of 66% on average, their actual adherence (in the previous 48 hours, as measured by drug concentrations) fell from 65% to 20% during the study. These levels are well below the adherence levels required for PrEP to be really effective.

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This again highlights the difficulties of PrEP as a general method for HIV prevention, though it may be of use in some special circumstances.