PrEP, an acronym from Pre-Exposure Prophylaxis, is the use of prescription drugs by people who do not have HIV/AIDS as a strategy for the prevention of HIV/AIDS. It is an optional treatment which may be taken by people who are HIV negative, but who have substantial, higher-than-average risk of contracting an HIV infection.
Currently, the only drug which any health organization recommends for PrEP is Truvada, which is the brand name of the Gilead Sciences drug combination of tenofovir/emtricitabine. The Centers for Disease Control says that "PrEP is a powerful HIV prevention tool and can be combined with condoms and other prevention methods to provide even greater protection than when used alone. But people who use PrEP must commit to taking the drug every day and seeing their health care provider for follow-up every 3 months."
One of PrEP's active ingredients, tenofovir disoproxil (TDF), is a nucleotide analog reverse transcriptase inhibitor. TDF effectively blocks HIV from incorporating its genetic material into the host's genome, and thus prevents HIV infection. 
PrEP is intended for use with condoms, so that each method can compensate for essential or casual efficacy deficits of the other. There are social groups which both support and oppose the use of PrEP.
In the United States, federal guidelines recommend the use of PrEP for HIV negative people with the following characteristics:
- in a serodiscordant sexual relationship, meaning that the HIV-negative person regularly has sex with an HIV positive person
- anyone who is not in a monogamous relationship with an HIV negative person, and who...
- is a man who has sex with men, and...
- does not regularly use condoms during sex, and who has sex with people of unknown HIV status who are at high risk of contracting HIV
- anyone who has injected illicit drugs in the past six months, shared recreational drug injection equipment with other drug users in the past six months, or who has been in treatment for injection drug use in the past six months
Reasons to avoid
Reasons for not using PrEP include the following:
- Persons with HIV should never use PrEP, and an HIV test is necessary before starting to use PrEP
- Persons with kidney problems, especially decreased renal functions, have increased safety problems with using PrEP
- Persons with hepatitis B have increased safety problems with using PrEP
- Women who are pregnant or breastfeeding should speak with their doctors about potential risk to their children
- Minors may not have access to services which complement the effective use of PrEP, and need extra attention from their doctor if they use PrEP
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. Any deleterious effect of Truvada on kidney function usually reverses with drug discontinuation, but irreversible kidney damage can rarely occur.
The terms "pre-exposure prophylaxis" and "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 16 July 2012. The CDC amended its guidelines for HIV prevention recommending pre-exposure prophylaxis with Truvada to high infection risk populations on 14 May 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 feel enabled to be more promiscuous and less safe in their sex practices due to a perceived heightened sense of security from H.I.V. infection because of the drug. Overall, the fight against HIV infection seems to be going strong. Infection rates have fallen 28% overall from 2007 to 2012.
In July 2014 the World Health Organization issued guidelines saying it "strongly recommends men who have sex with men consider taking antiretroviral medicines as an additional method of preventing HIV infection." 
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 analysis of the data has suggested that 99% protection is achievable if the drugs are taken every day.
In December 2015, the IPERGAY study was published looking at an alternative strategy of "on-demand" PrEP where Truvada was taken 2-24 hours before sexual activity and only continued for 2 days afterwards. In a population of 400 gay men in France and Canada at high risk for HIV, this strategy led to an 86% drop in HIV infections over the average 9 month follow-up of the study. As of December 2015, non-continuous PrEP methods have not been endorsed by WHO or national guidelines.
|Study||Type of PrEP||Study Population||Efficacy||Percent of patients who took medication (adherence)|
|CAPRISA 004||Pericoital tenofovir gel||South African females||39% reduction of HIV infection||72% by applicator count|
|iPrEx||Oral emtricitabine/tenofovir||Men who have sex with men||42% reduction of HIV infection. 99% reduction estimated with daily adherence||54% detectable in blood|
|Partners PrEP||Oral emtricitabine/tenofovir; oral tenofovir||African heterosexual couples||Reduction of infection by 73% with Truvada and 62% with tenofovir||80% with Truvada and 83% with tenofovir detectable in blood|
|TDF2||Oral emtricitabine/tenofovir||Botswana heterosexual couples||63% reduction of infection||84% by pill count|
|FEM-PrEP||Oral emtricitabine/tenofovir||African heterosexual females||No reduction (study halted due to low adherence)||<30% with detectable levels in blood|
|VOICE 003||Oral emtricitabine/tenofovir; oral tenofovir; vaginal tenofovir gel||African heterosexual females||No reduction in oral tenofovir or vaginal gel arms [oral emtricitabine/tenofovir arm ongoing]||<30% with detectable levels in blood|
|Bangkok Tenofovir Study||Oral tenofovir||Thai male injection drug users||48.9% reduction of infection||84% by directly observed therapy and study diaries|
|ANRS - Ipergay||Oral emtricitabine/tenofovir||French gay males||86% reduction of infection (Video summary)||86% with detectable levels in blood|
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- PrEPWatch PrEP Watch homepage
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- Aidsmap.com: Nevirapine studied as pre-exposure prophylaxis for HIV: safe in 200 mg dose
- Aidsmap.com: Pre-exposure prophylaxis may only offer limited benefit
- CDC: CDC Pre-Exposure Prophylaxis (PrEP)