PrEP vs PEP vs DoxyPEP: What's the Difference?
One of the things I see most often in my work is confusion around PrEP, PEP, and DoxyPEP. They sound similar, they’re sometimes used together, but they do very different things — and mixing them up can leave you under-protected without realising it.
I’m Dr George Forgan-Smith, a general practitioner with specialist training in HIV and sexual health, based in Melbourne. I want to walk you through each of these so you know exactly what they’re for, when to use them, and how to talk to your doctor about getting started — including how DoxyPEP and PrEP fit together, and why so many people get DoxyPEP vs PrEP confused.
At a glance
- PrEP → prevents HIV, taken before sex
- PEP → prevents HIV after a risky exposure — within 72 hours
- DoxyPEP → prevents chlamydia and syphilis, taken after sex
PrEP — preventing HIV before exposure
PrEP stands for pre-exposure prophylaxis. The clue is in the name: you take the medication before sex to prevent HIV infection.
There are a few ways to take it:
- Daily PrEP — one tablet every day.
- PrEP on demand — tablets taken around the time of sex, on a specific dosing schedule. There’s some nuance to this one, and it isn’t suitable for everyone — particularly people living with hepatitis B, or for cis and trans women, where the recommendations differ.
- Injectable PrEP — currently two options exist worldwide: cabotegravir (every two months) and lenacapavir (every six months). Both are available in Australia but aren’t yet covered by the PBS, so they’re expensive out of pocket. In the US, some insurance plans cover the cost. Talk to your doctor about what’s accessible where you live.
Important: PrEP only prevents HIV. It does nothing for chlamydia, gonorrhoea, syphilis, or any other infection.
Side effects
Most people tolerate PrEP very well. A small number get some nausea, bloating, or constipation when they first start — this almost always settles within a couple of weeks on daily dosing. If you’re using PrEP on demand and the gut side effects are bothering you, switching to daily for a while can help you get past that initial adjustment.
There’s a rare side effect of reduced kidney filtration. In more than ten years of prescribing PrEP, I’ve only had to take one patient off it for this reason — and even then, we have alternatives like Descovy that work well for people with reduced kidney function. Your kidney function is checked every three months while you’re on PrEP, so any change is picked up early.
For injectable PrEP, the main side effect is local — some pain or discomfort at the injection site.
PEP — the rescue treatment
PEP is post-exposure prophylaxis. This is what you take after a significant HIV exposure to try to prevent infection taking hold.
You’d consider PEP after:
- Condomless insertive or receptive sex with someone whose HIV status you don’t know
- A condom break
- Sharing needles
- A needlestick injury
- Sexual assault
- Missing PrEP doses around the time of a risk exposure
PEP is a 28-day course of one or two tablets, depending on the level of risk involved.
The critical thing to know about PEP is the time window. It’s only effective within the first 72 hours of exposure, and the earlier you start, the better it works. If something happens in the middle of the night, on a weekend, or you can’t get a GP appointment quickly — go straight to your nearest emergency department or sexual health service. Don’t wait.
If you’ve ever needed PEP, that’s a strong signal you should be having a conversation about PrEP. Being prepared is so much easier than being reactive — and clinically, PrEP is far simpler to manage than scrambling for PEP after the fact.
What is DoxyPEP?
DoxyPEP is a different beast altogether. It’s a single 200mg dose of doxycycline — a common antibiotic — taken within 72 hours after sex to prevent bacterial STIs, primarily chlamydia and syphilis. The name stands for doxycycline post-exposure prophylaxis.
It’s been a particularly important development for gay and bisexual men, where rates of chlamydia and syphilis have been climbing for years. The major trials were run in this population, and current Australian guidelines recommend DoxyPEP specifically for gay, bisexual, and other men who have sex with men, and trans women, who’ve had a bacterial STI in the past 12 months. That said, it’s appropriate for anyone at ongoing risk of bacterial STIs — your doctor can help you work out whether it fits your situation.
Does DoxyPEP stop chlamydia?
Yes — and very effectively. DoxyPEP reduces chlamydia infections by roughly 70–90% in the major studies, and it’s similarly effective against syphilis. It’s much less reliable against gonorrhoea, partly because of growing antibiotic resistance, so don’t count on it for that.
Does DoxyPEP prevent HIV?
No. DoxyPEP does nothing for HIV. This is the single most common misunderstanding I see — people on DoxyPEP assuming they’re covered against HIV as well. They aren’t. Doxycycline is an antibiotic and only works on bacterial infections; HIV is a virus. If you want protection against both HIV and bacterial STIs, you need both PrEP and DoxyPEP.
How long after sex can I take DoxyPEP?
You have a 72-hour window after sex to take your DoxyPEP dose, but sooner is better — ideally within the first 24 hours. After 72 hours its protective effect drops off significantly, so don’t try to “make up” a missed dose later in the week.
How to take DoxyPEP
DoxyPEP is straightforward but there are a few things worth getting right.
- Dose: 200mg of doxycycline (usually two 100mg tablets) as a single dose after sex.
- Timing: within 72 hours of sex, ideally sooner.
- Frequency: you don’t need to take it every day. During a busy stretch, dosing every three days is plenty — that’s around two doses a week at most. This reduces side effects and avoids overloading your system with antibiotic.
- With water, upright: wash the tablet down with a large glass of water and stay upright for at least 20 minutes afterwards.
DoxyPEP is particularly worth considering for people who’ve had recurrent chlamydia or syphilis. If you’ve had more episodes than you’d like over the last year or two, it’s worth a conversation with your doctor.
DoxyPEP side effects
For most people, DoxyPEP is very well tolerated. The main things to be aware of:
- Stomach irritation and DoxyPEP nausea. Doxycycline can irritate the lining of the stomach and oesophagus, which is what causes the nausea or queasiness some people report. The water-and-upright trick above prevents almost all of this. If you’re still getting nausea, try taking the dose with a small amount of food (not dairy, which reduces absorption).
- Sun sensitivity. Doxycycline can make your skin more sensitive to UV. Sunburn while you’re taking it can become unusually red, inflamed, and sore — so be a bit more careful with sun protection than you might normally be.
- Thrush or gut upset. As with any antibiotic, some people notice a bit of thrush or loose stools, particularly if they’re dosing more frequently.
Is DoxyPEP safe long term?
This is a fair question, and one I get often. The short answer is: based on what we know so far, yes — but we’re still building the long-term evidence.
The two main concerns clinicians watch for are antibiotic resistance (particularly in gonorrhoea and in gut bacteria) and impacts on your microbiome. Current evidence suggests that intermittent dosing — the way DoxyPEP is meant to be taken, not daily — keeps both risks low. The benefits in reducing chlamydia and syphilis are substantial, especially for people who’d otherwise be getting these infections repeatedly.
This is exactly why DoxyPEP works best when it’s prescribed and reviewed by a doctor who knows what they’re doing, rather than self-sourced. Regular three-monthly sexual health checks are part of the package.
Putting it together
Here’s how the three compare side by side:
| PrEP | PEP | DoxyPEP | |
|---|---|---|---|
| What it prevents | HIV | HIV | Chlamydia & syphilis (less effective for gonorrhoea) |
| When you take it | Before sex | After a significant exposure | After sex |
| Time window | Ongoing — before exposure | Within 72 hours of exposure | Within 72 hours of sex |
| How long you take it | Continuously while at risk | 28 days | Single 200mg dose, no more than 2× per week |
| Form | Daily tablet, on-demand tablets, or injection (2- or 6-monthly) | Tablets (1 or 2, depending on risk) | Doxycycline tablet |
| Best for | Anyone at ongoing risk of HIV | Emergency / rescue use | People with recurrent bacterial STIs |
| Does NOT cover | Bacterial STIs, hepatitis | Bacterial STIs | HIV, gonorrhoea (only partial) |
PrEP and DoxyPEP work well together — one covers HIV, the other covers the most common bacterial STIs. PEP is what you reach for when something has gone wrong and you need protection retrospectively.
Whichever of these you’re on — or even if you’re on all three at different times — please keep up your regular sexual health screening every three months. None of these medications cover everything, and routine testing is what catches what slips through.
Talking to your doctor
If you’re not sure how to start the conversation, keep it simple and direct:
“I’m thinking about PrEP — I’d like to reduce my risk of HIV.”
“I don’t always use condoms. Can we talk about PrEP and DoxyPEP?”
“I keep getting STIs and it’s frustrating. Is DoxyPEP something I could try?”
If your usual doctor isn’t familiar with these treatments, or you don’t get a warm response, find a doctor who does this work regularly. Gay health clinics and sexual health services are excellent. You don’t have to be gay to access them — you just need a clinician who does sexual health day in, day out.
PrEP is for anyone at risk of HIV. DoxyPEP is for anyone at risk of bacterial STIs. These aren’t niche treatments — they’re standard preventive medicine, and I’d much rather help someone start PrEP than treat them for HIV, which is a lifelong condition.
If you have any questions, leave them in the comments below. Stay safe.
Frequently asked questions
Can I take DoxyPEP and PrEP at the same time?
Yes — and for many people, that combination is exactly the right approach. PrEP and DoxyPEP work on completely different things (HIV vs bacterial STIs) and don’t interact in any clinically meaningful way. Taking both gives you broad coverage: PrEP handles HIV, DoxyPEP handles chlamydia and syphilis. Most of my patients on DoxyPEP are also on PrEP.
What’s the difference between DoxyPEP vs PrEP?
PrEP is taken before sex to prevent HIV. DoxyPEP is taken after sex to prevent bacterial STIs (chlamydia and syphilis). They’re not alternatives to each other — they’re complementary, and many people benefit from being on both.
What’s the difference between PrEP vs PEP vs DoxyPEP?
- PrEP — preventive, taken before sex, protects against HIV.
- PEP — emergency rescue treatment after a high-risk HIV exposure, must be started within 72 hours, taken for 28 days.
- DoxyPEP — single antibiotic dose after sex, prevents chlamydia and syphilis (not HIV).
Does DoxyPEP prevent gonorrhoea?
Only partially. Effectiveness against gonorrhoea is much lower than against chlamydia or syphilis, mostly because of antibiotic resistance. Don’t rely on DoxyPEP for gonorrhoea protection — keep up regular three-monthly testing so any infection gets picked up and treated early.
Do I still need regular STI checks if I’m on DoxyPEP and PrEP?
Yes, absolutely. Every three months. No combination of medications covers everything, and routine testing is what catches what slips through.
Dr George Forgan-Smith is a Melbourne-based GP with specialist training in HIV and sexual health. This article is general information and isn’t a substitute for individual medical advice — please speak to your own doctor about what’s right for you.