Q and A


Is PEP effective if taken 48 hours after exposure?

I paid for sex with a woman but the condom tore and I only realised after I was done. There is a high chance that she was HIV positive. I started PEP after 48 hours.

I got a PCR test done 17 days after exposure. It had a less than 20/ml (undetectable) result.

Now between 14 and 35 days after exposure I have had signs of thrush, mild red rash on my face and a couple elsewhere. But no fever or swollen lymph glands.

Can the rash and thrush be because of utter stress and obsession?

My doctor says that the PCR can be relied upon and that I am negative.

Can the PEP have reduced the viral load to less than 20/ml as I took it for 15 days before the PCR test?

Please give your expert advice. I am very anxious. Thank you.


Thank you for your question.

The risk of HIV from one broken condom is very low – probably only 1 in 500 is your partner was actually HIV positive and not on treatment. If your partner had an undetectable viral load on treatment, then the risk will be zero.

You cannot guess someones HIV status so your assumptions might also be wrong.

PEP will have reduced any risk further, but you need to wait about a month after PEP before testing.

PEP – stands for Post-Exposure Prophylaxis. It is a term used for taking HIV drugs after a possible exposure to reduce the risk of infection. After sexual exposure, PEP usually involves taking a combination of three HIV drugs for one month. The earlier PEP is started, the more likely it could work. Ideally, this should be within a couple of hours. If this is not possible, then still the earlier the better.

Some guidelines include a two-day window to do this (i.e. within 48 hours). After this, effectiveness is much less likely. Other guidelines allow up to 4 days (i.e. within 72 hours). Sometimes this may be more for psychological benefit (if someone is very stressed or traumatised) than because of any likely effectiveness.

No guidelines recommend any benefit from taking PEP after longer than 72 hours.

If the PEP has worked, or the person was not initially infected, this will show as a negative HIV test at one month and three months after the PEP treatment finishes. According to the information you have given us, it appears that you did a test 17 days after the exposure, rather than after PEP finished.

If the PEP has not worked, this will be shown in a positive test result. Testing 4 weeks after PEP will detect 95% of infections. Testing three months after PEP will detect more than 99.99% of infections and is considered confirmation that someone is HIV negative.

If PEP has not worked, serosonversion usually occurs 1-3 weeks after PEP is finished, although only 80% of people show these symptoms.

The best way to confirm whether the PEP has worked is therefore to test a month after PEP is finished. you then need to follow up with a confirmatory test three months after you complete the PEP.

Please note that viral load tests are not approved to test for HIV but they can sometimes have a roll.

The risk of infection from a single exposure is very low. Even if your partner was HIV positive, this might be less than 1 in 500. The symptoms you describe therefore, could be due to any number of reasons. The best way to confirm what is causing them is to see your doctor. He/she will be able to diagnose and treat the symptoms accordingly.

Note: This answer was updated in January 2018 and September 2014 from an original post from December 2011.


  1. Roy Trevelion

    Hi Erastus,

    What type of test did your girlfriend have? Please see here for the Q&A about different types of tests for HIV.

    Was the positive result for an antibody only HIV test? This does not mean that you are HIV positive. It means you need a second confirmatory test. So it is possible that it was a false positive.

  2. Erastus

    Mine is a concern about my fiancee who is internship in kids session.it happen that the kid and mother were both positive and needle pricked my fiance and am worried she tested while on pep and the results came out positive.Am very stressed since we didn’t even have a kid of our own.kindly advice me if this could be false positive

  3. Roy Trevelion

    Hi Ric,

    I hope you don’t mind but I’ve shortenend your comment. As Angelina points out, the risk of HIV from one broken condom is very low – probably only 1 in 500 if your partner was actually HIV positive and not on treatment.

    If your partner had an undetectable viral load on treatment, then the risk will be zero.

  4. Ric

    I wish to add my opinion and findings, PEP or PrEP if taken with accuracy diligently is very effective.

  5. Roy Trevelion

    Hi Jt,

    The BASHH guideline sounds straightforward. However, PEP can delay HIV infection. And it’s also important to start taking PEP as soon as possible after exposure to help make it effective.

    So there are factors that might make saying that a paticular time or date is absolutely conclusive.

  6. Jt

    There are many people on this site who say a test at 90 days after the last dose of pep is conclusive. But the latest BASHH guideline say a test 8-12 weeks post exposure with a 4th generation test is conclusive. Can someone please clear this up for me. When is testing conclusive when taking pep? 12 weeks after exposure or 12 weeks after last dose of pep?



  7. Lisa Thorley

    Hi Tj,

    Testing after a course of PEP can be done 4 weeks late, so 8 weeks after a possible exposure. If you’ve tested negative at 12 weeks this means that you don’t have HIV.


Your email address will not be published. Required fields are marked *