Bottom vs top

CDC says either partner can get HIV during anal sex, but being the bottom, with your partner’s penis inside your rectum, makes you more likely to get it. Per act, with a partner who has HIV and isn’t on treatment, the estimates are:

  • Receptive anal sex (bottom): 1.4% (about 1 in 72)
  • Insertive anal sex (top): 0.11% (about 1 in 910)

For comparison, receptive vaginal sex is 0.08% (about 1 in 1,200) per act. These are averages: the first time can transmit HIV, and many acts may not.

Between men

When two men have sex and the partner’s status is unknown, the main thing that changes the estimate is how common HIV is among men who have sex with men. UNAIDS estimates that gay men and other men who have sex with men are about 23 times more likely to acquire HIV than the general population. The calculator uses that figure automatically for sex between men.

For one act without a condom with a man of unknown status:

  • Bottoming, United States: 0.05% (about 1 in 2,000)
  • Topping, United States: 0.004% (about 1 in 25,000)
  • Bottoming, United Kingdom: 0.01% (about 1 in 9,800)

Circumcision lowers men’s risk of getting HIV through vaginal sex, but a Cochrane review found the evidence for men who have sex with men inconclusive. Read more in our guide for gay and bisexual men.

Between a man and a woman

The calculator uses the same per-act figure for any receptive partner. For a woman having receptive anal sex with a man who has HIV and isn’t on treatment, it’s 1.4% (about 1 in 72), much higher than for vaginal sex with the same partner.

With a man of unknown status in the United States, a woman’s estimate for one act of receptive anal sex is 0.0024% (about 1 in 42,000). For a man having insertive anal sex with a woman of unknown status there, it’s 0.00019% (about 1 in 530,000).

What lowers the risk

  • Condoms. In real-world use, including the occasional slip or break, condoms lower risk by about 80%. Bottoming with a condom and a partner who has HIV: 0.28% (about 1 in 360).
  • PrEP. PrEP (pre-exposure prophylaxis) is medicine that people without HIV take to prevent it. Taken as prescribed, it lowers the risk from anal sex by about 99%. Bottoming on consistent PrEP with a partner who has HIV: 0.014% (about 1 in 7,200). Options include daily pills, event-driven pills for some men who have sex with men, and long-acting injections. See PrEP and PEP.
  • Treatment. A partner with HIV who keeps an undetectable viral load doesn’t pass HIV on through sex. See U=U.
  • No ejaculation inside. The calculator lowers the bottom’s estimate by 50% when the top didn’t ejaculate inside.

What raises it

  • Other STIs. Sores and inflammation from infections like syphilis, gonorrhea or herpes make it easier for HIV to get in. The calculator multiplies the estimate by 2.
  • A recent infection. In the first weeks after someone gets HIV, their viral load peaks, often before they know. The calculator raises the estimate 10 times.

What to do now

Bottomed or topped without a condom in the last 72 hours? If your partner has HIV and isn’t on effective treatment, or you don’t know their status, ask about PEP (post-exposure prophylaxis) now at an emergency department, urgent care center or sexual health clinic. It must start within 72 hours, ideally within 24. How to get PEP

  1. Act on PEP first if you’re inside the 72-hour window. Don’t wait for a test result. PEP is taken daily for 28 days.
  2. Test after the window period. A NAT (nucleic acid test) can detect HIV from 10 days. A lab antigen/antibody test (blood from a vein) gives a conclusive negative from 45 days. If you take PEP, expect follow-up tests about 4 to 6 weeks and 12 weeks after the exposure. See test accuracy by week.
  3. Get an STI check. Anal sex can pass on other infections too, and having one raises HIV risk next time.
  4. Plan ahead. If you have anal sex without condoms, or with partners whose status you don’t know, PrEP is worth asking about: it’s protection you control yourself.