Having 10 or more sexual partners over a lifetime is linked to a heightened risk of being diagnosed with cancer, according to new research published in the journal BMJ Sexual & Reproductive Health.

However, it's not as simple as "more sex = more cancer" – it's a hazy and complex connection that needs further exploration to clarify, so stay tuned before jumping to any conclusions. 

The study by Anglia Ruskin University in the UK found that men who reported having 10 or more sexual partners in their lifetimes were 69 percent more likely to have a cancer diagnosis, while women with 10 or more sexual partners were 91 percent more likely to develop cancer. Despite this, there was no link between the number of sexual partners and their general health, nor their risk of heart disease or stroke. 

The researchers analyzed a dataset of 2,537 men and 3,185 women participating in the English Longitudinal Study of Ageing, a nationally representative study of adults over 50 in England. They also controlled for a range of demographic and health-related factors, such as age, ethnicity, partnership status, socioeconomic status, smoking status, alcohol intake, physical activity, and depressive symptoms.

Around 28.5 percent of men said they had had 0 or 1 sexual partners to date, 29 percent said they had had 2 to 4, 20 percent said 5-9, and 22 percent reported 10 or more. For women, just under 41 percent reported 0 or 1 sexual partners, 35.5 percent said 2 to 4, under 16 percent reported 5 to 9 partners, and just under 8 percent reported 10 or more.

The research was an observational study and did not specifically look to explain this link between more sexual partners and the risk of cancer. However, they hint that sexually transmitted infections (STIs) might be the link. For example, human papillomavirus (HPV) is responsible for around a third of all cancers caused by an infection, contributing to cervical cancer and penile cancer, as well as cancers of the mouth, throat, and anus. Chronic hepatitis B and C can also increase the risk of liver cancer.

While an increase in the number of sexual partners is generally linked to an increased risk of STIs, there are a bunch of other factors that could affect a person’s risk of being infected with an STI.

Importantly, the study's authors acknowledge a number of caveats and limitations to the research. First of all, the study didn’t look at the specific types of cancer participants reported. Secondly, it’s noteworthy that people with higher numbers of sexual partners also tended to live and behave differently to those with lower numbers: they were more likely to smoke and drink frequently, but they were also more likely to exercise. Lastly, it’s not possible to say why women appear to experience a greater risk of cancer if they have over 10 sexual partners compared to men. 

So, while undoubtedly an interesting connection, it's best not to make any hasty judgments before we know more.

“Ultimately, this study raises more questions than it answers,” Jayne Luce, an Honorary Professor at the University of Queensland’s School of Public Health, commented in an article for The Conversation.

“The paper concludes by saying enquiring about lifetime sexual partners could be helpful when screening for cancer risk. This is a very long stretch based on the evidence presented. This approach could also be harmful. It could invade privacy and increase stigma about having multiple sexual partners or having an STI.”