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Pre-exposure prophylaxis (PrEP) is any medical or public health procedure used before exposure to the disease causing agent, its purpose is to prevent, rather than treat or cure a disease. An example would be if a doctor gave a medication used to treat a disease to a healthy person who is not thought to have that disease, but is at risk for contracting it. More specifically, this practice is common with people who are about to travel from an area without malaria to an area where malaria is a risk, and also it is being researched as a tool to prevent persons from contracting HIV.

Pre-exposure prophylaxis can also refer to the aggressive use of vaccination, for example in an attempt to prevent rabies in people such as laboratory workers who are high risk for being bitten by rabid animals.

PrEP for malaria
The CDC publishes recommendations for travels advising about the risk of contracting malaria in various countries.

Some of the factors in deciding whether to use chemotherapy as malaria PrEP include the specific itinerary, length of trip, cost of drug, previous adverse reactions to antimalarials, drug allergies, and current medical history.

PrEP for HIV
Most commonly, the term pre-exposure prophylaxis refers to an experimental HIV-prevention strategy that would use antiretrovirals to protect HIV-negative people from HIV infection. Along with AIDS vaccines and microbicides, PrEP is one of the HIV prevention strategies being tested in clinical trials today. With an estimated 39.5 million people living with HIV worldwide and 4.3 million new infections per year, many people are at risk for HIV infection through sexual transmission, and therefore preemptive measures must be taken to prevent further dissemination. Pre-exposure prophylaxis (PrEP) provides a promising prevention strategy for further HIV transmission.

Studies of PrEP strategies in non-human primates have shown 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. Another rationale of PrEP comes from strategies to prevent mother-to-child transmission, which use ARVs given to the mother and the infant to help reduce the risk of transmission.

PrEP Studies
The PrEP studies utilize the drug tenofovir or a tenofovir/emtricitabine combo (Truvada) that is delivered orally. Additionally, tenofovir can be used as a topical gel. PrEP has been shown to reduce the risk of HIV infection in four clinical trials while two trials showed no protective benefit. As of 2012, there are still two ongoing PrEP studies that have not published findings while additional open label extensions are currently under way.

Criticism of PrEP
There are several challenges to PrEP, from biomedical concerns (such as safety and effectiveness) to concerns of behavioral repercussions.

The PrEP studies have shown the drugs to be safe, with few side effects. Generally, any minor side effects of nausea or diarrhea resolve themselves within the first few months. It is important to note that effectiveness is limited by adherence to the drug. In iPrEx, persons with greater than 90% adherence to the drug had an efficacy of 68% protection versus persons with less than 50% adherence who had an efficacy of 16%. Furthermore, the FEM-PrEP trial that was stopped early due to futility, found that the women's adherence to the drug was too low to find any effect on reducing HIV infections. Given mounting evidence of the effectiveness of PrEP among different populations when the drug is taken correctly, concern has turned more towards the practicalities of implementing PrEP as prevention. Since approximately 60% of people needing anti-retroviral therapy are not getting it, there are concerns that trying to deliver PrEP to many more people would be challenging. It is also important to consider how programs might target those at highest risk of HIV exposure in order to provide PrEP. Additionally, concerns about behavioral repercussions are currently being addressed. For example, the CDC (Centers for Disease Control) is conducting an extended safety trial in the U.S. to determine if men who have sex with men taking the drug are more likely to engage in riskier behaviors; preliminary results do not show any change in behavior.