According to the NIH:

• Prostate problems
• Type 2 diabetes
• Hypogonadism in association with a number of endocrinologic conditions
• Hypertension (high blood pressure)
• Vascular disease and vascular surgery
• High levels of blood cholesterol
• Low levels of HDL (high-density lipoprotein)
• Chronic sleep disorders (obstructive sleep apnea, insomnia)
• Drugs
• Neurogenic disorders
• Peyronie’s disease (distortion or curvature of the penis)
• Priapism (inflammation of the penis)
• Depression
• Alcohol use
• Lack of sexual knowledge
• Poor sexual techniques
• Inadequate interpersonal relationships
• Many chronic diseases, especially renal failure and dialysis
• Smoking, which exacerbates the effects of other risk factors, such as vascular disease or hypertension