The study, conducted at the University College London School of Pharmacy, assessed the medical records of nearly 270,000 men newly diagnosed with erectile dysfunction, who didn’t have thinking or memory problems and whose average age was 59. Researchers tracked these patients over an average of five years, and adjusted data to account for smoking status, alcohol consumption, and other factors that could influence the odds of developing Alzheimer’s. The results, published this week in the journal Neurology, found that men who took erectile dysfunction drugs were 18% less likely to develop Alzheimer’s than those who didn’t.

What’s more, men who were issued the most prescriptions throughout the study had the greatest reduction in risk.

“This is an exciting starting point,” study coauthor Ruth Brauer, Ph.D., tells Fortune, while stressing that “an association does not mean a causal effect.”

How Viagra may protect against Alzheimer’s 

PDE5s work by relaxing blood vessels. These PDE5s are approved in the U.S. to treat erectile dysfunction, and all were included in the study:

• sildenafil (Viagra)
• tadalafil (Cialis)
• vardenafil (Levitra)
• avanafil (Stendra)

“These drugs, however, in animal models showed that the vasodilation, or the relaxation of blood vessels, extended to the brain,” Brauer says, referencing a 2019 review in the journal Behavioural Brain Research. “These animal models show that increased cerebral blood flow might actually be neuroprotective.”

Brauer and her colleagues felt similar observational studies so far conducted in humans were inconclusive and wanted to make their own contribution to the research. While the team’s results are promising, Brauer says additional studies—randomized controlled trials, in particular—are in order.

Researchers had to make a key assumption in their analysis: that the men prescribed the drugs consumed them as instructed.

“We can only see who is prescribed drugs. We have no information [on] who picks up the prescription from a pharmacy, and we don’t know who is actually taking it,” Brauer says. “If you are brave enough to go to the GP to ask for PDE5 inhibitors, you’re very motivated to take these drugs. So we assume that prescriptions translated into use.”

Can women take ED drugs to stave off Alzheimer’s?

PDE5s aren’t approved for use in women in the U.S. Furthermore, it would be unwise to have healthy women take such drugs with no guaranteed benefit, Brauer says.

Still, Brauer envisions future trials that would include patients of all genders who already are experiencing mild cognitive decline, with the hope that PDE5 drugs would slow the rate of decline. Men without a history of erectile dysfunction would be a key part of that equation.

While UCL researchers were able to take patients’ socioeconomic status into account, other factors, such as patients’ physical activity, were difficult to measure, Brauer says. 

Noting “ethnicity is poorly recorded in U.K. GP records,” Brauer adds that studying a more racially and ethnically diverse patient cohort would also help scientists better understand the association between PDE5s and the risk of developing Alzheimer’s.

For more on Alzheimer’s disease and dementia:

• Belly fat tied to brain health in middle-aged adults at high risk of Alzheimer’s, study finds—but it’s worse for men
• TV personality Wendy Williams receives identical diagnoses to Bruce Willis—frontotemporal dementia and primary progressive aphasia
• Will you develop dementia? These 11 factors are strong predictors in middle age, scientists say
• These 10 US states are predicted to have the most people with Alzheimer’s disease. Here’s why