If PDE5 drugs don't work or cannot be used because of potential side effects, your doctor can recommend other therapies. The drug alprostadil (Caverject, Edex, Muse) allows blood to flow more freely in the penis, leading to an erection. The drug can be injected with a tiny needle into your penis. Or, a small pellet (suppository) can be inserted into the opening of the penis. Suppositories and injections are effective in the majority of men.
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). 
It appears that testosterone has NOS-independent pathways as well. In one study, castrated rats were implanted with testosterone pellets and then divided into a group that received an NOS inhibitor (L-nitro-L-arginine methyl ester [L-NAME]) and a control group that received no enzyme.  The castrated rats that were given testosterone pellets and L-NAME still had partial erections, a result suggesting the presence of a pathway independent of NOS activity.
"A lot of the symptoms are mirrored by other medical problems," Hedges says. "And for a long time, we were not attributing them to low testosterone, but to diabetes, depression, high blood pressure, and coronary artery disease. But awareness and appreciation of low testosterone has risen. We recognize now that low testosterone may be at the root of problems."
Lifestyle choices that impair blood circulation can contribute to ED. Smoking, excessive drinking, and drug abuse may damage the blood vessels and reduce blood flow to the penis. Smoking makes men with atherosclerosis particularly vulnerable to ED. Being overweight and getting too little exercise also contribute to ED. Studies indicate that men who exercise regularly have a lower risk of ED.
Performance anxiety can be another cause of impotence. If a person wasn’t able to achieve an erection in the past, he may fear he won’t be able to achieve an erection in the future. A person may also find he can’t achieve an erection with a certain partner. Someone with ED related to performance anxiety may be able to have full erections when masturbating or when sleeping, yet he isn’t able to maintain an erection during intercourse.
Long-term predictions based on an aging population and an increase in risk factors (eg, hypertension, diabetes, vascular disease, pelvic and prostate surgery, benign prostatic hyperplasia, and lower urinary tract symptoms) suggest a large increase in the number of men with ED. In addition, the prevalence of ED is underestimated because physicians frequently do not question their patients about this disorder.
Although not proven, it is likely that erectile dysfunction can be prevented by good general health, paying particular attention to body weight, exercise, and cigarette smoking. For example, heart disease and diabetes are problems that can cause erectile dysfunction, and both are preventable through lifestyle changes such as sensible eating and regular exercise. Furthermore, early diagnosis and treatment of associated conditions like diabetes, hypertension and high cholesterol may prevent or delay erectile dysfunction, or stop the erectile dysfunction from getting more serious.
Late onset hypogonadism reflects a particular pathophysiology and it may not be appropriate to extrapolate results from studies concerning the effects of testosterone in treating hypogonadism of other etiology to aging males. For this reason, the age of men treated in clinical trials is certainly relevant. Other important factors include patient comorbidities and the preparation and route of testosterone replacement used in the study, which can affect the production of estrogen and dihydrotestosterone, testosterone’s active metabolites
The most common treatment for erectile dysfunction is drugs known as phosphodiesterase-5 (PDE-5) inhibitors. These include tadalafil (Cialis), vardenafil (Levitra), and sildenafil citrate (Viagra). These are effective for about 75% of men with erectile dysfunction. They are tablets that are taken around an hour before sex, and last between 4 and 36 hours. Sexual stimulation is required before an erection will occur. The PDE-5 inhibitors cause dilation of blood vessels in the penis to allow erection to occur, and help it to stay rigid. Men using nitrate medication (e.g. GTN spray or sublingual tablets for angina) should not use PDE-5 inhibitors.
Best of all? It's easy. "Low T Center is set up so men can walk in, take a simple blood test, and know within 30 minutes whether or not they are a candidate for testosterone replacement therapy, or TRT. Men who qualify get their first injection on the spot, and will continue to come in three times per month to receive a quick testosterone injection."