Do I Need PrEP If I’m A Top?

“I’m a top, so I’m fine.”

It’s one of the most durable bits of folk wisdom going around, and I hear it in clinic constantly. It’s half right, which is exactly what makes it dangerous.

Topping is lower risk than bottoming. It is not low risk.

The numbers, and the one line that makes sense of them

Here’s the risk of picking up HIV from a single episode of condomless anal sex.

What you’re doing	Risk per episode, if he has HIV
Bottoming, he cums	1 in 70
Bottoming, he doesn’t cum	1 in 155
Topping, you’ve got your foreskin	1 in 160
Topping, you’re circumcised	1 in 900

Per act with a partner who has HIV. Not per act with anyone.

Read those middle two rows again.

On the guideline numbers, an uncut top’s risk per episode (1 in 160) sits in the same ballpark as a bottom’s when the top doesn’t cum (1 in 155). The position isn’t protecting you anywhere near as much as you think it is. (How solid that cut/uncut split actually is, I’ll come to below. It’s shakier than the table makes it look.)

These are the Australian guideline numbers (ASHM, 2025), and they come from a Sydney study that followed 1,427 gay men in the early 2000s. ^{1 2}

Two things about that table you need before it means anything.

First, every row assumes your partner has HIV. If nobody in the bed has HIV, none of these numbers are your numbers.

Second, and this one surprises people: those figures aren’t broken down by his viral load. ASHM says so in the footnote to its own table. They come from a real-world mix of partners, some on treatment and some not, all averaged together into one number.

And ASHM’s own caveat, which I’d rather you heard from me than found later: these estimates “do not adequately estimate an individual’s risk after a single exposure.” ² They’re population arithmetic. They tell you the shape of the thing, not what happened to you on Saturday.

Viral load changes everything, and it isn’t in the table

If your partner is living with HIV and on treatment with an undetectable viral load, he cannot pass it on. That’s what undetectable equals untransmittable means, and it’s settled science. ASHM puts it in the same footnote: if the viral load is undetectable, there is no risk.

Viral load isn’t inside that table, and it isn’t a separate sum you do on top of it either. Here’s the honest version.

If a partner is undetectable, the risk is zero. Full stop, no arithmetic required.

If he’s got HIV with a detectable viral load, or nobody knows yet, the table figures are the closest thing we have. And because they’re an average that includes partners on treatment, they may well sit on the low side for that situation rather than the high side.

That’s the shape of it, and it’s about as precise as this gets. Anyone offering you a tidier number is making it up.

The good news underneath all this: in Australia most people living with HIV are diagnosed, on treatment, and undetectable. Most of the time you’re nowhere near that table.

Here’s the catch, and it’s the reason this page exists. You don’t know which situation you’re in. Plenty of people don’t know their status yet. That’s not a character flaw, it’s just how a virus with no symptoms works.

You can’t screen for it by looking. You can’t screen for it by asking, because he may not know either. What you can do is make the question stop mattering.

About that foreskin row

You’ll have noticed the table says 1 in 900 if you’re cut and 1 in 160 if you’re not. That’s a near six-fold gap, and I want to be honest with you about how solid it is, because it isn’t.

It comes from one Sydney cohort. The insertive subgroup rests on 7 infections. The confidence intervals around the two figures overlap heavily, which is statistician for “these two numbers can’t reliably be told apart”.

And when researchers pooled the global data (62 studies, 119,248 men), circumcision came out protective in low and middle income countries but showed no effect at all among men in high-income countries like Australia. ³

The proposed mechanism is real enough on paper: the inner foreskin is thinner and mucosal, with more of the immune cells HIV targets. But plausible mechanism plus one cohort isn’t the same as established fact.

So: ASHM publishes both numbers and I’ve given you both. I wouldn’t plan your sex life around being circumcised, I definitely wouldn’t get circumcised for it, and if you’re cut I’d hold that 1 in 900 loosely. It isn’t zero either way.

So how well does PrEP actually work for tops?

This is the question I get asked most, and it rarely gets a straight answer, so here it is.

PrEP doesn’t know what you’re doing in bed. It works by keeping enough drug in your system that HIV struggles to get established, wherever it turns up. Taken as prescribed, it lowers your chance of getting HIV by about 99%. ⁴

Now the honest bit: the trials were never big enough to give tops their own separate number, so I’m not going to invent one for you. There’s no reason to expect a worse result, and there’s certainly no separate “top version” of the tablet. But if you want a figure that says “PrEP is X% effective specifically for tops”, it doesn’t exist, and anyone quoting you one is guessing.

What is different is that a top’s baseline risk was lower to begin with. So PrEP takes a smaller number and makes it much smaller.

“But I’m on PrEP and he’s not”

This one comes up constantly, so let’s be clear about who PrEP protects.

Your PrEP protects you. Only you. It’s not a force field around the bed.

If you’re taking PrEP as prescribed and topping, your risk is very low. Your bottom’s isn’t.

If you’ve both got recent negative tests, that lowers the chance considerably. Two caveats worth knowing: a test can’t see an infection picked up in the few weeks before it, and it only speaks for the day it was taken. It’s HIV only, too. But if the situation is reversed and he’s the one who might be exposed, your PrEP does nothing for him.

The flip side is the reassuring one: if he’s living with HIV and undetectable, he can’t transmit to you regardless of what you’re taking. Your PrEP is doing its work with everyone else, not with him.

On-demand suits a lot of tops

Worth knowing if you’re not having sex every week.

Australian guidance (ASHM, updated 2025) makes on-demand PrEP an option for cisgender men whatever the sex of their partners. Tops often have the kind of sex you can see coming, and that’s exactly what on-demand dosing is built for.

Daily is simpler if you’d rather not plan. The medicine itself is on the PBS either way.

One thing your doctor will raise: the 2-1-1 schedule sits outside the daily dosing the medicine is formally registered for in Australia, so it’s prescribed off-label, in line with ASHM guidance. That’s normal and it’s well supported, but it’s a conversation to have with your doctor rather than a decision to make alone.

One thing PrEP won’t do for you

Being a top doesn’t protect you from the rest of it either.

Urethral gonorrhoea and chlamydia are very much a top’s problem, and syphilis doesn’t care which end you’re on. PrEP covers HIV and nothing else, so the 3-monthly screen is still doing real work. More in does PrEP protect against other STIs.

If it already happened

If you topped condomless in the last 72 hours and you’re worried, this is the part that’s time-critical.

PEP is a month of tablets that can stop HIV establishing itself after an exposure. It works best the sooner you start, and 72 hours is the outer limit, not the target. Go to a sexual health clinic or an emergency department today, not next week. In Australia you can ring 1800 PEP NOW (1800 737 669) to find your nearest service.

If it’s been longer than 72 hours, PEP has missed its window, but that’s a reason to book a test rather than a reason to sit with it. And if you get a fever or a flu-like illness in the weeks after a possible exposure, get seen and say why.

The bottom line

Tops get skipped in most sexual health advice, and it shows. Topping is lower risk than bottoming, and it is not low risk. If you’ve got a foreskin, the guideline number puts you in the same range as the bottom you’re with.

Those numbers only apply when a partner has HIV, and they say nothing about his viral load. You often won’t know either. PrEP is how you stop having to guess.

Stay safe, team.

Tops get skipped elsewhere too. I’ve written about the specific causes of tops’ erection problems that nobody names.

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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