[Question #9539] PrEP Missed Doses

31 months ago

Dear Experts,

I have been on PrEP for several years now, in addition I usually use condoms however on Sunday morning I had unprotected insertive anal sex with a random man. We had sex about three times that morning. He is on PrEP and states that he tested negative several weeks ago and has not had any encounters since then. The week leading up to the encounter, I believe that I missed two doses of PrEP (non-consecutively.) I had taken the Descovy for sure on T,W,F,Sat, Sun and every day since then and I believe that I took it last Thursday but unsure and I believe that I missed it last Monday. After counting my prescriptions I believe that I took Descovy 30/33 days but unsure when my missed doses were (although I believe that two were in the 10 days or so leading up to this encounter). Overall I believe that I am pretty adherent to the medication with occasional missed doses. In addition earlier in the month of December I was taking fiber supplements (never at the same time as the medication) which caused me constipation, could constipation cause decreased absorption of Descovy? Further, I believe that the man was tested several weeks ago but if he was in the window period at that time and if he has a mutant strain of HIV that bypassed his PrEP what would be my risk in this situation? If he had acquired HIV while on PrEP would he be able to transmit the virus to me who is on PrEP. If he was infected with a mutant strain would that render PrEP useless, or would you expect that his viral load would be lower due to being on PrEP and less likely to transmit. Thanks

Edward W. Hook M.D.

31 months ago

Welcome to our Forum. Thanks for your questions. I'll be glad to comment.

Congratulations on your commitment to safe sex. PrEP will virtually eliminate your risk for acquisition of HIV although missed doses, particularly if missed consecutively which fortunately is not the case for you, may somewhat increase risk for infection if exposed. Missing two doses 4 days apart however would, at most increase your risk slightly if at all. Further, in that you'd taken your PrEP on the Friday and Saturday before your exposure, it should be virtually fully effective. Further, the fact that your partner stated he too was on PrEP further reduces the likelihood that he was infected (most people tell the truth). Thus, as I see it, your PrEP was effective at the time of your exposure and it is unlikely your partner had HIV. In the unlikely circumstance that he was infected at the time of your exposure, the estimated risk of three exposures is less than 3/1000ths (in other words, changes were greater than 99% that you would not be infected). Bottom line, I would not be concerned if I were in your situation.

Your fiber supplement and resulting constipation would not be expected to reduce the effectiveness of your PrEP.

There is no good answer to your "what if" question about a mutant strain that he been not prevented by his PrEP, sorry. Too many unknowns here. Rather, I would presume it ito be most unlikely that you were infected.

I hope these comments are helpful. EWH

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