Brief Answer:
Most common cause is either atherosclerosis or diabetes.

Detailed Answer:

Hi,

Thanks for posting your query.

I have gone through your query and understand your concern towards causes of erectile dysfunction.

The most common cause could be the conditions resulting in decreased blood supply to the penis.

Those are usually diabetes and atherosclerosis that is clogged blood vessels.

I would suggest get all routine blood investigations and ECG, chest X-ray.

Hope this information would help you.

Take care.

Dr. Soujanya, Psychiatrist

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