Lead author Dr Ruth Brauer said: "More research is needed to confirm these findings, learn more about the potential benefits and mechanisms of these drugs and look into the optimal dosage."
The researchers also want to run a trial in women as well as men, to see if the drug has any impact.
There are many factors which could be causing the disease. The study adjusted its findings for some of them, including age, underlying health conditions, other medicines taken and whether the participant was a smoker.
"This study does not conclusively prove that erectile dysfunction drugs reduce Alzheimer's risk but provide good evidence that this type of drug is worth further study in future," said Prof Tara Spires-Jones, from the University of Edinburgh, and president of the British Neuroscience Association.
Dr Francesco Tamagnini, neurophysiologist at the University of Reading, said it was "a great study", but more hard evidence on how the drug affected the brain was needed.
"It could be that it exerts a therapeutic effect directly affecting neurons (if the drug is able to cross the blood brain barrier) and/or by increasing blood flow, but both these hypotheses need to be tested," he said.