There is a lot that goes on during sex that can impact vaginal flora, from bacteria in incoming saliva, semen and vaginal fluids to the type of lube and condoms/barrier used.

The state of the vaginal microbiome before the interaction also matters. If flora is a bit weak in terms of protective species present, protective bacteria can be easily overwhelmed by pathogens already present or introduced into the vagina.

A healthy vagina is robust and can easily tolerate lots of sex, changes in pH and the incoming bugs of a lover. Problems can develop for a few reasons.

If you’ve found that you’ve not fared well vaginally after sex, here are some reasons why.

1. Incoming pathogens

There are two very common situations that deliver ‘non-STI’ pathogens directly into the vagina during sex: partner genitals (even if they don’t have symptoms), and anal to vaginal penetration or ‘mixed play’.

Penises and vaginas can easily carry pathogenic bacteria or yeast and deposit them directly into each new vagina they encounter. Reinfection then occurs with every subsequent interaction without both partners being treated unless condoms/barriers are always used. Mouths, fingers and toys can also be a source of transmission.

These imported pathogens are not considered sexually transmitted infections (STIs), despite plenty of evidence of partner transmission and resultant vaginal infections. Pathogens might already be present or be transmitted without necessarily causing issues, while our classic STIs almost always cause problems and are only sexually transmitted.

If you’ve had microbes transmitted, you may develop symptoms of bacterial vaginosis, aerobic vaginitis, and sometimes, a yeast infection. Then you have your regular STIs, which a protective microbiome may or may not protect you from.

2. Hypersensitivity to semen, lube or condoms/barriers

Many people are allergic to latex or sensitive to ingredients in lubricants, and another smaller proportion of people are allergic to the proteins in semen. All three can create inflammation in the vagina, which protective species of bacteria do not enjoy.

Bacteria usually live on the outside of cells, and an inflamed cell isn’t a nice place to live for protective lactobacilli. Some pathogens, the cockroaches of the bacterial world, can live just about anywhere and will happily take over a vagina given half a chance. An inflamed vagina is not a happy vagina.

Thankfully, removing the offending items from rotation tends to clear up the issue, but that gets a little complicated for semen-allergic people who are trying to conceive.

3. A weak vaginal microbiome

If the vaginal microbiome is already low on protective flora, menstrual bleeding or semen can allow pathogens, already present or introduced, to overgrow.

The causes of a weak vaginal microbiome are many and varied, but if you consider our gut and vaginal microbiomes to be a reflection of what’s going on in the rest of the body, you can understand how small shifts in the vagina (sex) can leave you open to developing symptoms.

If we’re unwell, stressed, have hormonal imbalances or use vaginal products or douches that modify or rinse away healthy flora, the protective vaginal microbiome may not be very strong.

Getting a comprehensive microbiome panel and then seeking support to help rebuild protective colonies - and keep them there - is a solid approach. Support in this sense might be, for example, correcting or managing underlying health issues or problematic food choices rather than just direct vaginal treatments. Everyone is a bit different in this respect.

4. pH changes due to periods, semen and alkalising lube

Protective lactobacilli species produce lactic acid and hydrogen peroxide, which acidifies the vagina and repels pathogens. The vagina itself is not acidic, and therefore the pH of the vagina can be modified by different types of bacteria.

In someone who is still getting periods, a pH usually reflective of the presence of protective lactobacilli is at about 4.0-4.5 on the pH scale (remembering that 1 is extremely acidic,7 is neutral, and above that is alkaline).

Some pathogens, in particular those that cause bacterial vaginosis, do not produce lactic acid, and therefore if the pH rises, it can reflect colonisation with non-lactobacilli species.

But, a vagina with non-lactobacilli species isn’t necessarily bad. In menopause and children’s vaginas, for example, a pH of 5.5 is the norm, as oestrogen is low and food for all bacteria decreases.

Some pathogens also produce lactic acid, so pH is used as a general guide only.

Semen and menstrual blood are both alkaline, so the influence on the vagina is alkalising. A stable, protective microbiome won’t flinch at this - the base is strong and healthy and won’t be that easily swayed by pH changes.

If the vagina, however, is low in protective species, the increased pH and energy sources provided by semen or blood can create a little food festival that allows pathogens to multiply rapidly.

Some lubricants designed for supporting conception are deliberately alkaline, as sperm need an alkaline environment to swim to the egg. Most lubricants are either pH balanced or, in the case of silicone, have no pH value as they do not contain water.

Tips to protect the vaginal microbiome during sex

• Get regular comprehensive microbiome tests (annually) to see your starting point, and if your microbiome is a little weak, seek support to improve it
• Use a form of barrier protection like condoms or dental dams during sex
• Never ever put anything into a vagina after it’s been in an anus
• Don’t let anyone with bad breath or smelly genitals near your vagina
• Avoid sharing fluids via fingers and toys if you’re with a new partner you’re not sure about
• If you start having unprotected sex with someone, check in with your partner about their history regarding their previous partners known to have developed urogenital problems (BV, UTIs), as it may be an indicator that they are the carrier
• Don’t treat unnecessarily - if you have a healthy vaginal microbiome, relax and enjoy it!