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Pre-exposure prophylaxis 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. It is also commonly used as a tool to prevent persons from contracting HIV, as is recommended by the CDC.
Some of the factors in deciding whether to use chemotherapy as malaria pre-exposure prophylaxis include the specific itinerary, length of trip, cost of drug, previous adverse reactions to antimalarials, drug allergies, and current medical history.
The terms "pre-exposure prophylaxis" or "PrEP" most commonly refer to an HIV-prevention strategy where antiretrovirals are used to protect HIV-negative people from HIV infection. The HIV antiviral Truvada was approved by the FDA for PrEP on July 16, 2012. The CDC amended its guidelines for HIV prevention recommending pre-exposure prophylaxis with Truvada to high infection risk populations on May 14, 2014,  due to research indicating prophylactic effectivity preventing transmission from mother to child. Prior to that date, Truvada was only approved to treat existing HIV infections.
PrEP has been gaining a lot of attention and focus, especially within the battle against AIDS in New York City, a metropolis known for the epidemic's fatality. Governor Andrew Cuomo of New York has pledged his battle to end AIDS by 2020. According to a New York Times article, Cuomo intends to dramatically drop the infection number to 750 by 2020 from about 3000 in 2013 and 14,000 in 1993. In a city that has a large gay population, Cuomo intends to target his gay male audience. In addition to the use of condoms, Cuomo hopes that PrEP will continue to reduce the number of infections. However, the drug has met some sort of resistance as users may be noted as "Truvada whores," or people who miss their daily uptake of the drug and still do not use a condom. Overall, the fight against HIV infection seems to be going strong. Infection rates have fell 28% overall from 2007 to 2012. 
Most PrEP studies utilize the drug tenofovir or a tenofovir/emtricitabine combination (Truvada) that is delivered orally. 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 study of the data indicated 99% protection with daily adherence.
|Study||Type of PrEP||Study Population||Findings|
|CAPRISA 004||Pericoital tenofovir gel||South African females||39% reduction of HIV infection|
|iPrEx||Oral emtricitabine/tenofovir||Men who have sex with men||42% reduction of HIV infection. 99% reduction estimated with daily adherence|
|Partners PrEP||Oral emtricitabine/tenofovir; oral tenofovir||African heterosexual couples||73% and 62% reduction of infection|
|TDF2||Oral emtricitabine/tenofovir||Botswana heterosexual couples||63% reduction of infection|
|FEM-PrEP||Oral emtricitabine/tenofovir||African females||No reduction (study halted due to low adherence)|
|VOICE 003||Oral emtricitabine/tenofovir; oral tenofovir; vaginal tenofovir gel||African females||No reduction in oral tenofovir or vaginal gel arms [oral emtricitabine/tenofovir arm ongoing]|
|Bangkok Tenofovir Study||Oral tenofovir||Thai male injection drug users||48,9% reduction of infection|
The PrEP studies have shown the drugs to be safe, with few side effects. Generally, minor side effects such as nausea or diarrhea resolve themselves within the first few months. Effects of Truvada on kidney function have been shown to be temporary.