England’s health technology assessment body, The National Institute for Health and Care Excellence (NICE), has 'found that Truvada is clinically effective in reducing HIV acquisition in higher risk people but also identified issues around treatment that need to be considered, such as uptake and adherence, sexual behaviour and drug resistance. 'According to a 2012 Science Translational Medicine study, Emtricitibine / Tenofovir PrEP is able to reduce HIV infection via sexual transmission by 99% if it is taken daily, though it must be noted that if the patient doesn't take it as regularly, this can impact it's effectiveness. Similarly, a 2011 Science Translational Medicine study found that Truvada is also an effective medication for reducing the risk of HIV transmission in intravenous drug users by upto 74% (for people with detectable tenofovir levels). 'PrEP reaches maximum effectiveness after seven days for anal sex and 21 days for vaginal sex or injection-drug use'. However if doses are skipped or other preventative measures are not used up until the seven day mark (e.g. unprotected sex), it is much less effective, according to Very Well Health.