Basics

Note: as often raised at end of consultation, listen and then suggest a blood test be done and return so can assess properly

Organic vs psychogenic causes but often both
Loss of morning/overnight erections, gradual onset, loss of self-stimulated erections, normal libido more suggestive of organic cause
Organic causes include
Vascular – vascular risk factors, diabetes. Vascular most common cause of ED and high correlation with presence of atherosclerosis in coronary arteries.
Structural – peyronnie’s
Hormonal – hypogonadism (eg Klinefelter), thyroid disorders, hyperprolactinaemia. Routine testosterone not required and slightly low levels are often due to obesity. 
Neurogenic – pelvic/prostatic surgery, central or spinal neurological disease

Examination – should check genitalia (small testes) and also do DRE if > 50 yrs or has LUT symptoms

Investigations – lipids, glucose. 9 am testosterone if symptoms of hypogonadism. Consider PSA, prolactin, TSH

Management includes lifestyle advice (stop smoking, lose weight).

Often both organic and psychogenic components but useful to treat organic first, once erections happening through self-stimulation, then if not when with partner, arrange psychosexual therapy.
Medical therapy: PDE5 inhibitors – if two ineffective, refer for alprostadil penile injections, penile vacuum pump, penile prosthesis.
PDE5 inhibitors
Cannot be used if high risk of cardiac event eg. unstable angina, recent stroke/MI, resting hypotension, severe CCF
Cannot be taken with nitrates or nicorandil. GTN cannot be taken within 24 hr of taking sildenafil or 48hr after tadalafil
Common side effects are flushing, headache, indigestion, nasal congestion and vision disturbance.
Should be taken on empty stomach (aborption blocked by fatty meals) and 1 hr prior to intercourse. Require sexual stimulation in order to work. Suggest taking tablet at 4pm, it will last until midnight. 

Tadalfil can be taken daily (5mg) or on demand (or both), and also improves LUTS.