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2 Missed PREP Truvada doses

Jul 13, 2014


Thank you for reviewing these questions! I have reviewed this forum for years -- has been a life saver.

I have two questions.

Background: I have been on PREP for 4 months without ever missing a dose. My last exposure with any degree of risk has been approximately two weeks before the situation I am describing.


This last weekend, when traveling, I forgot my bag and was on an odd schedule. I was unable to take two of my Truvada PREP doses two days in a row.

If I do not miss any more doses for the month, assuming I have been exposed to HIV, is my risk much higher?

I understand the efficacy of PREP decreases with missed doses, but the studies show much more risk if a 1 or more doses are missed WEEKLY. If I already had therapeutic levels in my blood, I imagine that I was not without the drugs in my blood for very long -- less than the period of time people are typically started on post exposure prophylaxis. And if I do not miss any more doses, then I will not have regularly missed doses and I am not sure how the studies will apply to me.


Should I follow up any sooner on lab work? Should I request a viral load or other kind of HIV test and when?


Response from Dr. Young

Hello and thanks for posting.

There's not much specific data on how missed doses affect PrEP effectiveness, but we know that HIV treatment requires adherence around 90% or greater for optimal response. Missing one dose per week calculates out at 26/30 or 87%- probably lower than ideal. Missing more than 1 dose/week only increases these risks.

What's probably more important is the relationship of the missed dose to any possible HIV exposure. The effect of the missed dose would likely be greatest if it occurred in the 24-48 hours prior to, or after the exposure.

Perhaps a pill box or other reminder system can help you with your adherence-- or, in the future, a once-monthly PrEP injection.

Hope that's helpful, BY

replacing AZT with Isentress

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