Prostaglandins (alprostadil): Alprostadil can be injected into the penis or inserted as a pellet through the urethra. It causes an erection without sexual stimulation that usually lasts about 60 minutes. The danger with this method is that too high a dose can cause priapism, an erection that won't go away. This condition requires immediate medical attention as it can cause serious bruising, bleeding, pain and permanent penile damage. Once the doctor is sure of the right dose, the man can self-inject at home.
Stiffy Solution: Obviously, no one should ever go off a prescribed med without consulting with a doctor. But there are erection-friendly alternatives to nearly every daisy-wilting medicine listed above. And your dude shouldn't let embarrassment keep him from talking to his GP about this — doctors are well aware that erectile difficulties are a possible side effect for all of these medicines; they just don't know which patients will experience what side effects, so they're waiting for you, the patient, to bring it up.
Both ED and low testosterone (hypogonadism) increase with age. The incidence of the latter is 40% in men aged 45 years and older.  Testosterone is known to be important in mood, cognition, vitality, bone health, and muscle and fat composition. It also plays a key role in sexual dysfunction (eg, low libido, poor erection quality, ejaculatory or orgasmic dysfunction, reduced spontaneous erections, or reduced sexual activity). 
ED usually has something physical behind it, particularly in older men. But psychological factors can be a factor in many cases of ED. Experts say stress, depression, poor self-esteem, and performance anxiety can short-circuit the process that leads to an erection. These factors can also make the problem worse in men whose ED stems from something physical.
Erections occur in response to tactile, olfactory, and visual stimuli. The ability to achieve and maintain a full erection depends not only on the penile portion of the process but also on the status of the peripheral nerves, the integrity of the vascular supply, and biochemical events within the corpora. The autonomic nervous system is involved in erection, orgasm, and tumescence. The parasympathetic nervous system is primarily involved in sustaining and maintaining an erection, which is derived from S2-S4 nerve roots.