If your partner is living with HIV, or you’re unsure of their status, several proven strategies can reduce your risk of getting HIV to near zero. The most effective approach combines multiple layers of protection, but even a single method like PrEP or consistent condom use dramatically lowers your risk. Here’s what works, how well it works, and how to put it into practice.
Know the Baseline Risk
Not all sexual activity carries the same level of HIV risk. Without any form of protection and assuming an HIV-positive partner who is not on treatment, the CDC estimates these per-act risks:
- Receptive anal sex: about 1 in 72
- Insertive anal sex: about 1 in 909
- Receptive vaginal sex: about 1 in 1,250
- Insertive vaginal sex: about 1 in 2,500
These numbers represent a single encounter. Over months or years of repeated exposure, cumulative risk rises significantly. That’s why consistent protection matters more than any one-time decision. It’s also worth noting that other sexually transmitted infections, particularly those causing sores or inflammation (like syphilis, herpes, gonorrhea, or chlamydia), can multiply HIV transmission risk dramatically. Genital ulcers alone can increase the chance of transmission by a factor of 10 to 300, depending on direction. Staying on top of STI screening and treatment is a meaningful part of HIV prevention.
Your Partner’s Treatment Is Your Protection
If your partner is HIV-positive and taking antiretroviral therapy consistently, their viral load can drop so low that standard lab tests can’t detect it, typically below 20 copies per milliliter of blood. At that level, they cannot pass HIV to you through sex. This principle, known as Undetectable = Untransmittable (U=U), is backed by large clinical trials and years of observational data involving thousands of serodiscordant couples (where one partner has HIV and the other doesn’t). Zero transmissions have been documented when the positive partner maintained an undetectable viral load.
The key word is “maintain.” Viral load can fluctuate if your partner misses doses, changes medications, or develops drug resistance. Regular viral load monitoring, usually every few months, confirms that treatment is working. If your partner shares their lab results with you or you attend appointments together, you’ll have a clearer picture of where things stand.
PrEP: A Pill or Injection for the Negative Partner
Pre-exposure prophylaxis (PrEP) is medication you take before potential exposure to HIV. When taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. Two main forms are available.
Daily oral PrEP involves taking one pill every day. It’s straightforward and well-studied. For people who find daily pills hard to stick with, an injectable form is now available. The injectable version starts with two shots one month apart, then continues with an injection every two months. It’s administered by a healthcare provider and removes the need to remember a daily pill entirely.
There’s also an “on-demand” option called the 2-1-1 schedule, studied primarily in men who have sex with men. You take two pills 2 to 24 hours before sex, one pill 24 hours after the first dose, and one more pill 24 hours after that. This can work well for people who have sex infrequently and prefer not to take a daily medication, though it requires planning ahead.
PrEP requires a prescription and periodic follow-up visits, including HIV testing every three months to confirm you remain negative.
Condoms Still Work
Consistent, correct condom use reduces the probability of HIV transmission per act by up to 95%. Among serodiscordant couples who use condoms every time, annual HIV incidence drops by 90 to 95%. When condom use is inconsistent, protection falls to around 69%, which is still substantial but leaves more room for risk.
The distinction between “consistent” and “inconsistent” matters enormously here. Using condoms most of the time still provides real protection, but using them every time nearly eliminates risk on its own. Condoms also protect against other STIs that would otherwise increase your vulnerability to HIV, making them a useful layer even if you’re already using PrEP or your partner is undetectable.
Layering Methods for Near-Zero Risk
Each of these strategies is powerful on its own. Combined, they make HIV transmission virtually impossible. A common approach for serodiscordant couples looks like this: the positive partner stays on treatment and maintains an undetectable viral load, the negative partner takes PrEP, and the couple uses condoms. Any two of these three strategies together already brings risk extremely close to zero. All three together provide maximum peace of mind.
Which combination works best depends on your circumstances. Some couples rely entirely on U=U once the positive partner has been undetectable for months. Others prefer the added security of PrEP, especially early in a relationship or if there’s any concern about medication adherence. There’s no single right answer, just different comfort levels.
PEP: Emergency Protection After Exposure
If a condom breaks, PrEP was missed, or an unexpected exposure occurs, post-exposure prophylaxis (PEP) is a 28-day course of HIV medication that can prevent infection after the fact. PEP must be started within 72 hours of exposure, and the sooner, the better. Every hour counts. If you think you may need PEP, go to an emergency room or urgent care clinic immediately rather than waiting for a regular appointment.
PEP is not a substitute for ongoing prevention. It’s meant for emergencies. The medication must be taken every day for the full 28 days to be effective.
Regular Testing Keeps You Informed
If your partner is HIV-positive, routine testing for yourself helps catch any unlikely breakthrough early. Most people in serodiscordant relationships test every three to six months. If you’re on PrEP, testing every three months is already built into the follow-up schedule.
The type of test matters when timing is important. A lab-based antigen/antibody blood draw can detect HIV as early as 18 days after exposure. A rapid finger-stick version of the same test has a similar early window but may take up to 90 days to be conclusive. Rapid antibody-only tests and home self-tests generally need 23 to 90 days. Nucleic acid tests (NATs), which look for the virus directly, can detect infection as early as 10 days after exposure but are less commonly used for routine screening.
If you’ve had a specific exposure you’re worried about, ask for a NAT or lab-based antigen/antibody test rather than a rapid antibody test, since those detect infection sooner.
Treating Other STIs Matters Too
One of the most overlooked aspects of HIV prevention is managing other sexually transmitted infections. Both ulcerative STIs (syphilis, herpes, chancroid) and inflammatory STIs (gonorrhea, chlamydia, trichomoniasis) increase susceptibility to HIV by creating breaks in skin or triggering immune responses that make it easier for the virus to take hold. Modeling studies have estimated that preventing just 100 cases of syphilis in high-risk groups could avert 1,200 HIV infections over a decade.
Routine STI screening, prompt treatment, and open communication with your partner about symptoms all reduce this amplifying effect. If you’re already using PrEP, your regular follow-up visits are a natural time to screen for other infections as well.