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. Lisa Thorley

    Hi Tonny,

    When someone is on PEP they need to take the full course of medication, then wait 28 days to test. It won’t help to test midway. For info about what an Elisa test can and can’t do please see here: https://i-base.info/guides/testing/appendix-3-how-hiv-tests-work

  2. Tonny

    I am on my 14th day while taking PEP, I did an HIV Elisa test and it showed a viral load 40pcs.what are the chances of PEP clearing the virus after the 28day course.

    Can I transmit HIV while on PEP?

  3. Roy Trevelion

    Hi John,

    HIV can not be transmitted if your partner has been taking HIV treatment (ART) and has an undetectable viral load. Please see U=U. So if this is the case, there’s no risk of HIV.

    However, it’s a good idea to continue with PEP and take the complete course. Please see this link full info about PEP.

  4. John

    I am on my second week on PEP, now I had an unprotected sex with an HIV woman. Should I continue with my medication or I start again? Are there any chances that I might contract hiv

  5. Roy Trevelion

    Hi Worried Man,

    It’s recommended that you have an HIV test 28 days after finishing PEP because PEP can delay HIV infection. So it’s great that you’ve tested negative.

    For more info about PEP and PrEP, please see this link.

  6. Worried Man

    Hello, finished 30 days of PEP and tested negative. This is after a sexual encounter with a HIV positive lady. Is the test result conclusive?

  7. Lisa Thorley

    Hi Roy,

    PEP needs to be taken within max 72hrs after an exposure, the sooner the better.

  8. Roy

    Is Pep effective at the 49th hour post exposure?

  9. Roy Trevelion

    Hi John,

    I hope you don’t mind but I’ve shortened your comment a bit.

    It’s great that you started PEP if you thought this was a risk for HIV. Please see this link for all info about PEP. This info includes having another HIV test 28 days after finishing PEP.

    Please talk to your doctor if you think you are allergic to these HIV meds. That’s to make sure that if you need to take them again it will be okay.

    And please see the following links for reported side effects for these meds.
    Raltegravir: https://i-base.info/guides/3567
    Truvada: https://i-base.info/guides/746


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