How does Finasteride cause ED when it does?

The NHS wrote: This US study looked at a medical records database to see how common erectile dysfunction (impotence) was among men prescribed two drugs, dutasteride and finasteride, both used to treat non-cancerous prostate enlargement. The drugs work by blocking the male hormone testosterone. A low dose of finasteride is also used to treat male pattern baldness.

Overall they found that around 1 in 17 of all men prescribed either drug for prostate enlargement had erectile dysfunction. This figure fell to 1 in 31 of those prescribed finasteride for baldness. Using the drug for longer was generally linked with a higher risk. However, in 99% of men, stopping the drugs solved the problem so it wasn’t as catastrophic as the media implies.