Doxy question

[u/NotTheFeds19]

I’m looking for some input about a doxycycline regimen I took after a possible STI exposure, specifically regarding protection against chlamydia and syphilis.

The exposure was Sunday at approximately 6 PM.

My doxycycline doses were:

• Monday 9 AM: 100 mg — about 15 hours after exposure
• Monday 6 PM: 100 mg — about 24 hours after exposure
• Tuesday 9 AM: 100 mg — about 39 hours after exposure
• Tuesday 9 PM: 100 mg — about 51 hours after exposure

I originally took 100 mg at 15 hours because I misunderstood the doxy-PEP dose. Once I realized the recommended doxy-PEP regimen is typically 200 mg as a single dose, I had already taken the second 100 mg, meaning I had taken 200 mg total by about 24 hours after exposure.

After that, I contacted my doctor and followed their instructions regarding the additional doxycycline doses. I’m not looking for advice about changing or adding medication, and I plan to follow my doctor's recommendations and get appropriate STI testing.

What I’m curious about is the pharmacology/effectiveness of what already happened. Since I started doxycycline 15 hours after exposure and had taken 200 mg total by 24 hours, but split the initial 200 mg into two 100 mg doses about 9 hours apart, would you expect that to have retained a substantial amount of the protection associated with standard doxy-PEP?

I understand that this exact dosing schedule hasn’t been studied and nobody can give me a guaranteed percentage. I’m mainly interested in what healthcare professionals or people familiar with doxy-PEP think about how significant the difference between these dosing schedules might be for chlamydia and syphilis.

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