Saw palmetto is a naturally occurring and fairly potent 5-alpha reductase inhibitor, so it blocks the conversion of Testosterone into DHT, a stronger androgen that plays a large role in the male reproductive system during puberty and development and in adulthood it has metabolic functions, binds with androgen receptors to regulate gene transcription, and generally maintains sexual organ/tissue health and function. Finasteride and Dutasteride are common examples of drugs that are 5-AR inhibitors and yes, all of these including saw palmetto are notorious for causing ED due to the fact DHT is drastically lowered with continued use. DHT is the active androgen involved in the Nitric oxide production which is directly responsible for your ability to get an erection.