The vagina is home to a community of bacteria, and in many people it's led by a group called lactobacilli. They help keep the vagina acidic, which is the reason vaginal pH stays low on most days. That community isn't fixed. It shifts with the cycle, with hormones and with what comes into contact with it, and sex is one of the most regular of those contacts.
This article looks at what research has actually measured about sex and the vaginal microbiome: what semen does, why new partners matter, what condoms changed in one study, and what a recent trial found about treating partners. It also covers the habits around sex that help, the ones that don't, and when a change is worth having checked.
In this article
- The bacteria that live in the vagina
- What semen does
- New partners, more partners
- Why partners came into treatment
- What condoms changed in one study
- Habits around sex: what helps and what doesn't
- When to see someone
- Frequently asked questions
The bacteria that live in the vagina
When researchers describe a vaginal microbiome that looks settled, they usually mean one where lactobacilli are in charge. They're thought to keep other bacteria in check by making lactic acid and hydrogen peroxide. Studies often report a Nugent score too, a lab count that tells clinicians whether lactobacilli or other bacteria dominate a sample.
One species, Lactobacillus crispatus, comes up again and again in this research as the one most linked with a stable vagina. When lactobacilli lose ground, other bacteria can take over, and that shift is what's behind bacterial vaginosis, often called BV.
The vaginal pH article linked above explains the chemistry. This one stays with what studies have measured about sex and that balance.
What semen does
Semen doesn't share the vagina's acidity. Researchers who studied the vaginal flora of healthy women note that sperm has a pH of about 7.0 to 8.0, which is close to neutral and well above the vagina's usual range. After sex without a condom, that fluid sits in an environment it doesn't match, and it can push the pH up for a while.
A study of 426 women in Kenya, South Africa and Rwanda looked at which bacteria were present and how much. It found that recent sexual exposure was linked with less L. crispatus, less Lactobacillus vaginalis and fewer lactobacilli overall. The researchers measured this at one point in time, so the study shows a link with recent sex, not how long the change lasts.
Habits around sex count as well. The CDC names douching among the things that upset the normal balance of vaginal bacteria.
Other fluids and contact
Semen is the fluid studied most. Fingers, toys, saliva and lubricants also come into contact with the vagina, and the studies in this article didn't measure them. If you notice a pattern, such as an odour or discharge that shows up after sex more than once, mention it to a clinician.
New partners, more partners
Both the CDC and the NHS link BV with sexual activity, while stopping short of calling it an infection you catch the way you catch an STI.
The CDC says that douching, not using condoms, and having new or multiple sex partners can upset the normal balance of vaginal bacteria. It also notes that BV rarely affects people who have never had sex, and that you can't get it from toilet seats, bedding or swimming pools.
The NHS says BV is not classed as a sexually transmitted infection, but that sex can trigger it or pass it between partners. It lists having a new partner among the things that make BV more likely, along with being sexually active, having an IUD and using perfumed products in or around the vagina.
The African study above found a similar link. Having more than one sexual partner in the previous three months was associated with more Gardnerella vaginalis, one of the bacteria tied to BV.
Why partners came into treatment
For a long time, BV was seen as a problem belonging only to the person with a vagina. A trial published in 2025 changed that conversation.
Researchers in Australia enrolled couples in which a woman had BV and was in a monogamous relationship with a male partner. In one group only the woman was treated, while in the other her male partner also took oral and topical medication prescribed by the study team. The trial was stopped early because the difference was clear: BV came back in 35% of women whose partner was treated, against 63% in the group where he wasn't, within 12 weeks.
The authors began from evidence that bacteria linked with BV are exchanged between partners during sex. Their result supports that picture. It also means that if BV keeps coming back, asking a clinician whether your partner should be treated too is a reasonable question to bring to the appointment.
Two limits are worth knowing. The trial included only male partners in monogamous relationships, and the treatment was prescribed and followed by doctors, so none of it is something to try at home. The NHS's point that sex can pass BV between partners applies to partners of any sex.
What condoms changed in one study
Condoms keep semen out of the vagina, so researchers have asked whether they change the microbiome too.
A Chinese study of 164 healthy women aged 18 to 45 compared groups by the non-hormonal birth control they used. Women who used condoms consistently had more L. crispatus than women who used an IUD, and 93.1% of the condom group had a Nugent score in the healthy range. The authors concluded that consistent condom use may protect against BV.
That study looked at women at a single point in time, and it compared groups by the birth control they already used. So it shows an association, not proof that condoms caused the difference. It fits with the CDC's advice, which lists using condoms the right way every time you have sex among the steps that may lower the risk of BV.
| What happens during or after sex | What it can do to the vaginal microbiome | What to do |
|---|---|---|
| Semen in the vagina | Raises pH; recent exposure linked with fewer lactobacilli | Condoms keep semen out |
| A new partner, or more than one | Linked with more BV-associated bacteria | Notice changes in smell or discharge; see someone if they come back |
| Douching after sex | Upsets the normal balance of bacteria, according to the CDC | The CDC and NHS both advise against it |
| Perfumed washes, sprays or wipes | Linked with a higher chance of BV | Leave them out |
| Condoms used every time | Linked with more L. crispatus in one study | Supported by CDC advice, alongside STI protection |
| A partner treated for recurring BV | Fewer recurrences in a 2025 trial | Ask a clinician; never self-treat a partner |
Habits around sex: what helps and what doesn't
Most of what keeps the vaginal community steady around sex is about what you leave out.
Skip douching. It can feel like cleaning up, but the CDC names douching as something that upsets the vaginal balance, and the NHS advises against vaginal douches and washes. Our article on vaginal and rectal douching explains how the two differ.
Leave out perfumed products. The NHS lists perfumed products in or around the vagina as a factor that makes BV more likely, and it advises against vaginal deodorants and washes. This guide to washing the vulva covers what the outside needs and what it doesn't.
Choose your lube with care. A lubricant goes right where the bacteria live, so it's worth reading the label. Since the NHS links perfumed products in or around the vagina with BV, a lube with no fragrance is the gentler pick.
Keep toys clean, especially between partners. A shared toy can carry fluids from one person to the other. Washing it between uses, or covering it with a fresh condom when it's shared, keeps each person's fluids with that person. Our article on how to clean sex toys goes through materials and methods.
Use condoms if you're with a new partner. They protect against STIs, and the research above suggests they may help the microbiome too.
When to see someone
Many people with BV don't notice anything. The NHS says about half have no symptoms, and that it usually doesn't cause soreness or itching. When there are symptoms, the CDC describes a thin white or grey discharge and a strong fishy odour, especially after sex.
See a doctor, a nurse or a sexual health clinic if:
- You notice a new fishy smell or a thin grey or white discharge, especially if it's stronger after sex. BV usually needs antibiotics, according to the NHS.
- You're pregnant and notice those changes. The NHS advises getting treatment promptly in pregnancy, because there's a small risk of complications.
- It keeps coming back. That's when it's worth asking about your partner, based on the 2025 trial.
- There's itching, burning, sores or pain. The NHS says BV usually doesn't cause soreness or itching, so these need a proper look.
Smell changes have many causes, and not all of them involve bacteria. Our guide to why the vulva can smell different goes through the common ones and what's normal from one day to the next.
Frequently asked questions
Does sex change your vaginal pH?
It can, for a while. Semen has a pH of about 7.0 to 8.0, much higher than the vagina's usual acidic range, so sex without a condom can raise vaginal pH temporarily. A study of 426 women linked recent sex with fewer lactobacilli. It measured one point in time, so it can't say how long the change lasts.
Can a new sexual partner cause BV?
A new partner is linked with a higher chance of BV. The NHS lists having a new partner among the factors that make it more likely, and the CDC says new or multiple partners can upset the normal balance of vaginal bacteria. It doesn't mean anyone did anything wrong. If a new smell or discharge shows up with a new partner, a clinic can test for BV.
Is BV a sexually transmitted infection?
Not officially. The NHS says BV isn't classed as an STI, though sex can trigger it or pass it between partners. The CDC notes that it rarely affects people who have never had sex, and that you can't get it from toilet seats, bedding or swimming pools. It's an imbalance in the vagina's own bacteria.
Should my partner be treated if my BV keeps coming back?
Ask your clinician. A 2025 trial of monogamous couples found that BV returned within 12 weeks in 35% of women whose male partner was also treated, against 63% when only the woman was treated. The treatment was prescribed and followed by doctors, so it isn't something to try on your own.
Do condoms help the vaginal microbiome?
They may. A study of 164 healthy women found that consistent condom users had more Lactobacillus crispatus and more often had a healthy Nugent score than women using an IUD. It showed an association, not proof. The CDC lists using condoms correctly every time among the steps that may lower the risk of BV.
Should I wash or douche after sex?
Don't douche. The CDC names douching as something that upsets vaginal bacteria, and the NHS advises against vaginal douches, deodorants and washes. A gentle wash on the outside is all you need, and nothing goes inside. Douches and vaginal washes are exactly what both sources warn against.
Sources
- Healthy vaginal flora, condom use and L. crispatus, and the pH of sperm: Ma and colleagues, PLOS ONE, 2013, PMC3720897
- Recent sex, number of partners and vaginal lactobacilli in 426 women: Jespers and colleagues, BMC Infectious Diseases, 2015, PubMed 25879811
- Treating male partners to lower BV recurrence: Vodstrcil and colleagues, New England Journal of Medicine, 2025, PubMed 40043236
- BV risk factors, symptoms and prevention steps: CDC, About bacterial vaginosis
- BV and sex, things that make it more likely, and when to get treatment: NHS, Bacterial vaginosis
About this article
Written by the Peach & Cream team, based in Montréal. Published October 15, 2026. Last updated October 15, 2026. Facts come from the clinical sources and studies listed above. This article is general information, not medical advice.