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 Robs,

    It’s not easy to answer your question. Unfortunately, there is no data showing a difference between 21 and 28 days of PEP. Please see this Q&A.

    However, as Angeina says above, 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.

  2. Robs

    My partner and I were taking PEP for the last month he finished a week earlier than me. We had unprotected sex together and I’m now a bit worried about the outcome though we both tested hiv negative before starting PEP. Does this recent exposure increase the chances of getting HIV after 4days of completing PEP?

  3. Lisa Thorley

    Hi Julia,

    If there are trust issues, the easiest thing is to use PEP and then get tested.

  4. Julia

    Hy i sleep with someone i don’t know his status we use protection but the condom broke i don’t trust him so should i get infected

  5. Roy Trevelion

    Hi Skakes,

    Please see the reply below to your first comment.

    Here’s the guide to PEP

  6. Skakes

    i slept with an hiv position pregnant woman who is on treatment and the pregnancy is not mine…i started taking pep after 40 hours and now im having fever and night sweat please help

  7. Roy Trevelion

    Hi Skakes,

    It’s good that your partner is on HIV medication. That’s because it’s the best way to look after her health, and her baby’s health too.

    If she has been on treatment and now has an undetectable viral load, then the risk of HIV transmission is zero.

    Please see Undetectable = Untransmittable (U=U) here.

  8. Skakes

    i slept with an HIV positive pregnant woman who is on medication and the condom broke

  9. Roy Trevelion

    Hi Mr. Worry,

    Was the combo test you had 95 days after finishing PEP a 4th generation test?

    The window period for a 4th generation antigen/antibody test is about four weeks. By this time 95% of infections will be detected. There is a three month window period after exposure, for the confirmatory result to detect more than 99.9% of infections. Please see this link to window periods.

    So it looks like this result is more than 99.9% accurate.


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