availability, sex without condoms has increased. PrEP does not prevent the transmission of sexually transmitted infections other than HIV, and is not 100% effective. Barriers to use PrEP is underused. A systematic review of 76 peer-reviewed articles and 28 conference abstracts of values and preferences among global populations who might benefit from PrEP found that awareness of PrEP is low, but individuals were receptive to use when presented with information. Common barriers to PrEP use include lack of communication between an individual and their doctor, stigmatization, concerns about safety, side effects, and cost and effectiveness. Transgender women are disproportionally affected by HIV/AIDS, and PrEP is often underused. Barriers for PrEP use and access for transgender women include concerns about side effects, cost, hormone therapy, adherence, PrEP-related stigma and interaction with healthcare workers. Studies evaluating PrEP efficacy to reduce risk of HIV infection found a linear relationship between adherence and effectiveness of medication. This means that the more closely people follow recommended dosing of PrEP, the more effective the medication is at preventing infection. Research Most PrEP studies use the drug tenofovir or a tenofovir/emtricitabine combination (Truvada) that is delivered by mouth. Initial studies of PrEP strategies in non-human primates showed a reduced risk of infection among animals that receive ARVs prior to exposure to a simian form of HIV. A 2007 study at UT-Southwestern (Dallas) and the University of Minnesota showed PrEP to be effective in "humanized" laboratory mice. In 2008, the iPrEx study demonstrated 42% reduction of HIV infection among men who have sex with men, and subsequent analysis of the data has suggested that 99% protection is achievable if the drugs are taken every day. Below is a table summarizing some of the major research studies that demonstrated PrEP with Truvada to be effective across different populations. PrEP approaches with agents besides Truvada are being investigated. On December 20, 2021, the FDA approved cabotegravir (Apretude), which was the first injectable drug for PrEP that's taken every two months. There has been some evidence that other regimens, like ones based on the antiretroviral agent Maraviroc, could potentially prevent HIV infection. Similarly, researchers are investigating whether drugs could be used in ways other than a daily pill to prevent HIV, including taking a long-acting PrEP injection, PrEP-releasing implants, or rectally administered PrEP. However, it is important to keep in mind that , major public health organizations such as the U.S.