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. If your partner had an undetectable viral load on treatment, then the risk would be zero. If they were not on treatment the risk might still be only 1 in 300 or lower.

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

You started PEP within 48 hours, so it is likely it will work.

PEP – stands for Post-Exposure Prophylaxis. It is a term used for taking HIV drugs to reduce the risk of infection. PEP usually involves taking a combination of three HIV drugs for one month. The earlier PEP is started, the more likely it will 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. For example, if someone is very stressed or traumatised.

No guidelines recommend starting PEP after 72 hours.

After finishing PEP you need to wait a 4-6 weeks before testing.

If the PEP has worked, or the person was not initially infected, this will show as a negative HIV test. The test you did 17 days after the exposure will not be accurate yet.

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 after six weeks will detect more than 99% of infections. This result doesn’t need to be tested again.

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

Please test 6 weeks after you finished PEP is finished. If you test any earlier, you need to take a second test 3 months later.

Viral load (PRC) tests are not approved to test for HIV. In some cases they can have a role.

Please see a doctor to talk about your symptoms he 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 last updated in July 2021 from an original post from December 2011.


  1. Josh Peasegood

    Hi Peter, starting PEP so early means this result is likely accurate. But, for a test to be conclusive you will need to wait a few weeks for the result. If you use a 4th generation test it will be conclusive from 6 weeks after PEP. A 3rd generation test will require you to wait 12 weeks after PEP.

    You do not need to use both types of test to confirm your status.

  2. Peter

    Hello if someone had unprotected sex with an infected person but was put on pep 30hours after exposure and after 28days of taking pep he tested negative that was 2 days after completion of the medication is that test conclusive or their a possibility that the test after 2 -3 months will change help advise thanks

  3. Josh Peasegood

    Hi Richy, a 3rd generation test is conclusive from 90 days post PEP. Your result is very likely to stay the same but you will need to check in 10 days.

  4. Richy


    80 days Negative Post PEP using 3rd Generation Alere determine Kit..

    Can i take my mind off now


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