Everybody occasionally partakes in behavior that is potentially dangerous, whether this is intentional, such as casual unprotected sex, or unintentional, such as a workplace injury. The consequences of those actions can be scary, but perhaps the scariest of all consequences is transmitting a life-affecting virus such as HIV – and both of the previous examples could have this result. Of course, there are ways in which you can help protect yourself – avoiding risky situations where possible and following safety advice such as using condoms are a good start – but it’s impossible to avoid every potentially dangerous situation in life. Sometimes, risk of exposure to HIV is going to happen. So is there any way that transmission of HIV can be prevented after exposure? Perhaps surprisingly, the answer is yes. PEP, or post-exposure prophylaxis, is a treatment given to those who have been exposed to a high risk of HIV transmission and it’s used as a preventative medication in order to stop the virus in its tracks.

What is PEP?

PEP is essentially an antiretroviral (ARV) therapy made up of a combination of one to three ARV drugs and is actually the same therapy used to treat HIV once transmitted[1]. Originally only given to occupational exposures, such as needlestick injuries in health care workers, the US Department of Health and Human Services (HHS) offered guidelines in 2005 for the use of PEP in non-occupational (nPEP) cases such as exposure through sexual activity or drug use as well[2]. The treatment is given to those who test negatively for HIV and have been exposed to a high risk in a single case (ongoing exposure, such as a person with an HIV positive partner, is not normally treated with PEP, but with an alternative drug, PrEP – pre-exposure prophylaxis). In contrast to occupational PEP, nPEP is generally coupled with risk-reduction counselling and education in order to help patients learn from their damaging behavior. It is also only administered if treatment is sought within 72 hours of the exposure – any longer than that, and the treatment simply isn’t effective and therefore isn’t prescribed[3].

Window of Opportunity

There is a good reason for this seemingly tiny window of opportunity for treatment and it’s all to do with how the virus behaves for the first few days after entering the body. After initial contact, the virus infects the cells in and around the area of exposure (for example, the area at which the needle punctured the skin) and begins to replicate itself in order to create more HIV, known as virions. After two to three days, these virions move away from the initial point of exposure and spread around the rest of the body. It’s at this point that the infection becomes permanent and it’s before this point that preventative treatment needs to be administered[4]. The purpose of PEP, then, is to prevent the replication of cells and prevent the spread of infection, leaving those few exposed cells to die at the point of contact and thus, leaving the infection to die as well. So the science behind it is sound, but does it work?


It’s a hard thing to produce evidence for. Randomized tests with a control group and a placebo drug, such as are used for testing the majority of medical treatments, would not only be unethical in this case but also logistically almost impossible, so the evidence base for nPEP relies heavily on observational studies of both PEP and nPEP treatments. One such study was a review of medical records in New York and specifically, of new mothers with HIV and their babies. It was found that when PEP therapy was administered to mothers during labor or to the babies within 72 hours after birth, the risk of HIV transmission was reduced by 50%[5]. A similar study of needlestick injuries in health care workers shows that prompt administration of PEP reduced the risk of transmission by a huge 81%[6].

A further study took place in Brazil, where 200 homosexual and bi-sexual men were given four-day starter kits and instructions for PEP. Those who self-identified as high risk began taking the medication and after 96 hours, all were evaluated for incidences of HIV in blood serum. Of those who had begun the course of PEP, there was a 0.7% incidence rate and for those who hadn’t, there was a 4.1% rate. Once the course had finished, only 1 of those involved had contracted HIV whereas 11 of those not-involved had contracted HIV[7]. It’s not perfect then, but those seem like better odds than the alternative.

Perhaps one of the most convincing, albeit rather limited pieces of evidence in favor of PEP is the case study of one patient who had received a blood transfusion only to later discover that the blood received was in the early stages of HIV infection. A course of PEP treatment was begun a week later and continued for a full nine months. Despite the extremely high risk of contraction, PEP worked and the infection failed to develop[8].

Dangers of PEP

Like everything in life though, PEP is not perfect and it’s not without its problems. The efficacy of the treatment depends highly on how promptly it is initially administered but also on how well the treatment is adhered to (i.e. how well the patient follows instructions and maintains the course correctly) – and it’s not an easy course to adhere to. Normally lasting 28 days, PEP involves taking medication twice a day, taking frequent lab tests for things such as liver function, and then clinical follow-ups two, four, twelve, and twenty-four weeks after the initial exposure[9]. To make it worse, the treatment often has adverse side effects that make it even more difficult to adhere to. These can include prolonged headaches, diarrhea, nausea, and vomiting[10]. Evidence suggests that for those who stop taking PEP or don’t follow the course correctly, the primary cause is severe adverse effects, although luckily only 1.3% of people report having serious effects[11]. There are other problems as well, such as the virus building a resistance to the ARV medication – which, should PEP not work and the patient still contract HIV, can be extremely damaging to future treatments. It’s a pricey procedure too, and one that may or may not be covered by your insurance.

However, perhaps the most commonly used argument about the dangers of PEP is in regards to future behavior. Such a ‘quick fix’, some people argue, will encourage people to reduce their protective behaviors and continue or even increase their risky ones. Of course, it must be remembered that this is not a ‘quick fix’. As a treatment, there is no guarantee that it will work and surely slipping on a condom or grabbing a clean needle is significantly quicker than a 28 day course of painful medication and follow-up meetings for the next five months. Besides, reduction of risk behavior is at least part of the reason that prescriptions of nPEP are coupled with counselling sessions. What’s more, there is evidence to suggest that nPEP isn’t simply a slippery slope to wild behavior. A study in San Francisco found that of those given an nPEP treatment, 72% reported a drop in risky behaviors one year later[12].

PEP is not a perfect solution, that much is clear. Like all treatments, there are negative aspects and uncomfortable side effects. It’s not a quick fix and it is certainly not a substitute for safe sex. However, the evidence in favor of PEP is strong, and the Department for Health and Human Services suggests that “although data from the studies and case reports do not provide definitive evidence of the efficacy of nPEP after sexual, injection-drug-use, and other non-occupational exposures to HIV, the cumulative data demonstrate that antiretroviral therapy initiated soon after exposure and continued for 28 days might reduce the risk for acquiring HIV.”[13] So if you find yourself in the unfortunate position of having been exposed to a risk, seek medical help as soon as you can – remember, the sooner PEP is initiated, the more likely it is to work.


[1] Center For Aids, 2016, nPEP, Available at: http://www.centerforaids.org/nPEP.html

[2] Guidelines can be found at: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5402a1.htm

[3] Center For Aids, op cit.

[4] Harlon Davey, Laurel Challacombe, and James Wilton, 2015, Can we prevent infection with HIV after an exposure? The world of post-exposure prophylaxis (PEP), Available at: http://www.catie.ca/en/pif/fall-2010/can-we-prevent-infection-hiv-after-exposure-world-post-exposure-prophylaxis-pep

[5] Department of Health & Human Services, 2005, Antiretroviral Postexposure Prophylaxis After Sexual, Injection-Drug Use, or Other Nonoccupational Exposure to HIV in the United States, Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5402a1.htm

[6] Ibid.

[7] Ibid.

[8] Ibid.

[9] Center for Aids, op cit.

[10] NHS, 2015, Can post-exposure prophylaxis (PEP) stop me getting HIV?, Available at: http://www.nhs.uk/chq/Pages/1840.aspx?CategoryID=73

[11] Department of Health and Human Services, op cit.

[12] Department of Health and Human Services, op cit.

[13] Ibid.