Does PrEP Protect Against Other STIs?

Short answer: no. PrEP is built for one job, and that job is HIV.

Think of PrEP as a lock fitted to one door. It’s a very good lock. But the house has other doors, and PrEP doesn’t touch them.

What PrEP doesn’t cover

PrEP does nothing for chlamydia, gonorrhoea, syphilis, mycoplasma, hepatitis C, herpes, HPV, mpox, or shigella. Taken as prescribed, it lowers your chance of picking up HIV by about 99%, and that’s where its work stops.

This trips guys up, and I get why. “Protected” gets used as though it means protected from everything. It doesn’t. You can be rock solid on PrEP and still catch syphilis.

So should you use condoms?

Here’s where I’ll be straight with you, because most advice on this goes soft.

Plenty of guys start PrEP precisely because condoms weren’t getting used every time. Pretending otherwise helps nobody. So let’s talk about what condoms actually do rather than what we wish they did.

Condoms cut the risk of the infections that travel in fluid: chlamydia, gonorrhoea, HIV. They help less with the ones that spread skin to skin, because a condom only covers what it covers. Syphilis, herpes and HPV can all live on skin a condom never touches.

So condoms are a real tool, not a complete one. Use them where they work for you. If they don’t, that’s information rather than a failing, and it means the other layers matter more. There’s more in do I still need condoms on PrEP.

Where doxy-PEP comes in

This is the newer piece, and it’s the one most guys haven’t caught up on.

One thing to know upfront: doxycycline isn’t approved by the TGA for this use. Prescribing it this way is off-label, which is legal and common, but it means your doctor is making a judgement call with you rather than following a product’s registered indication.

Doxy-PEP is a single 200mg dose of doxycycline taken after sex, ideally within 24 hours and no later than 72.

In the trial that put it on the map (Luetkemeyer and colleagues, New England Journal of Medicine, 2023), run in gay and bi men and trans women who’d had a recent bacterial STI, it cut chlamydia by around 88% and syphilis by around 87%. Whether it does the same outside that group is less clear.

Gonorrhoea is the weak spot, at roughly 55% in that trial. The likely reason is tetracycline resistance, which is already common in gonorrhoea, though the study couldn’t prove that’s the whole story.

It doesn’t suit everyone, and it’s a conversation with your doctor rather than something you start off the back of a blog post. I’ve written the full doxy-PEP rundown covering who it’s for, the side effects, and the resistance question.

The screen is what actually catches things

Here’s the part I’d underline: most STIs give you nothing. No discharge, no sore, no symptom at all. Especially in the throat and the arse.

You won’t feel gonorrhoea in your throat. You can pass it on anyway.

Which is why the 3-monthly screen alongside your PrEP script is the layer guys most often treat as a formality, and the one that catches what everything else misses. Screening doesn’t stop you catching anything. It’s how you find what’s quietly sitting there, and treat it before it does damage or reaches someone else.

A full screen means swabs for chlamydia and gonorrhoea at every site you use (throat, arse, and the front), plus bloods for HIV and syphilis, with hepatitis C and your kidney function checked on the schedule your doctor sets. One site isn’t a screen. If you’re only handing over a urine sample, you’re missing most of it.

More on the 3-monthly review and what’s in it.

The layers you might have forgotten

Vaccines do quiet work in the background here. Hepatitis A and B, HPV, and mpox are all vaccine-preventable, and each is worth asking about at your next PrEP appointment if you’re not already covered.

And some things travel by routes nobody warns you about. Shigella spreads by tiny amounts of poo reaching the mouth, so rimming is the obvious one, but fingers, fisting and toys all count too.

When to get seen sooner than 3 months

The 3-monthly rhythm is for when nothing’s wrong. Don’t sit on any of these waiting for the calendar:

  • Discharge from your penis or front, your arse, or your throat
  • A sore, ulcer, or lump anywhere on your genitals, arse, or mouth
  • Rectal pain, or bleeding
  • Pain or burning when you pee
  • A rash you can’t explain, or fever
  • A message from a partner telling you they’ve tested positive for something
  • Missed doses around the time you had sex, or any worry you’ve been exposed to HIV while off your schedule

Any of those, book in now. Syphilis in particular can show up as a painless sore that heals on its own, which is exactly why people miss it.

That last one is the time-critical one. If you think you’ve been exposed to HIV and your PrEP wasn’t on board, PEP can still help, but the window is 72 hours and sooner is better. Don’t wait on it.

The bottom line

PrEP is excellent at exactly one thing. Build the rest of your cover around it: get the full screen every 3 months, get your vaccines sorted, and use condoms where they work for you. If you want to know whether doxy-PEP is worth considering in your situation, raise it at your next appointment.

That’s the whole picture, not just the HIV corner of it.

Stay safe, team.

This information is general in nature and not a substitute for personalised medical advice. Speak to your doctor about your specific situation.

Dr George Forgan-Smith, GP, practising in Sydney and Melbourne