Men who vape or use e-cigarettes are more than twice as likely to suffer from erectile dysfunction than those who have never used the popular nicotine-based smoking alternatives, a new study has found.

The study, which analyzed self-reported data from almost 14,000 American men aged between 20 and 65, found that regular e-cigarette users were around 2.2 times more likely to experience impotence than non-smokers. The findings were published in the American Journal of Preventive Medicine on Wednesday.

The risk was found to be 2.4 times greater in a smaller sample of men younger than 65 – with normal body mass indexes and no history of heart disease – when compared to non-vapers. The study did not include data on whether the respondents were taking medications that can raise the risk of erectile dysfunction, such as antidepressants or heart medication.

Researchers noted the “abundant evidence” already linking smoking and nicotine to sexual dysfunction and theorized that the particularly high level of nicotine in vaping liquid reduces blood flow to the penis and interferes with the normal functions of blood vessels, though they didn’t definitively prove the link.

Nonetheless, the study’s authors – from New York University and Johns Hopkins University – noted that vaping enthusiasts needed to be warned about the potential impact on their sexual health.

Citing research based on rodent models, the researchers also noted that another “plausible” link was exposure to e-cigarette “refill liquids with or without nicotine.” These contain chemicals that may reduce testosterone levels in the human body and impair normal erectile function.

“Overall, e-cigarettes are likely less harmful than smoking cigarettes to the degree that they substitute cigarette smoking,” the study’s lead author, Omar El Shahawy, told Insider. He added that men who view vaping as a healthier alternative should try to “limit” the habit as it is “simply not risk free.”

The study comes as England gears up to become the first country to prescribe e-cigarettes as a method to help smokers kick the habit. In October, the UK’s Medicines and Healthcare products Regulatory Agency issued updated guidance that would allow the products to be offered through the NHS.

But El Shahawy warned that the relationship between vaping and impotence needs to be “fully investigated” in further studies. The study did not detail whether the risk of erectile dysfunction was comparable to that posed by smoking.