Few simple laboratory tests can help identify obvious causes of organic ED. Initial labs should include HbA1c, free testosterone, thyroid function tests, and prolactin levels. However, patients who do not respond to pharmacological therapy or who may be candidates for surgical treatment may require more in-depth testing, including nocturnal penile tumescence testing, duplex Doppler imaging, somatosensory evoked potentials, or pudendal artery angiography.
Another oral treatment that has been used with very little success is yohimbine (Yocon, Yohimex). This is an alpha 2 adrenergic receptor blocker that increases cholinergic and decreases adrenergic tone. It stimulates the mid-brain and increases libido. Optimal results occur when used in men with psychogenic ED. Side effects include anxiety and insomnia.
A 2013 study published in The Journal of Sexual Medicine evaluated 439 men for erectile dysfunction and compared ED causes and frequency in men 40 or younger to men over 40. They found that 26 percent of the younger men had ED. Although these men were healthier and had higher levels of testosterone than the older men, they were more likely to be smokers or to have used illicit drugs. In almost half of the younger men with ED, the ED was considered severe.

Chronic and persistent alcohol use is known to induce sexual dysfunction, which leads to marked distress and interpersonal difficulty. This, in turn, is known to worsen the alcohol abuse. Sexual dysfunction in the alcoholic may be due to the depressant effect of alcohol itself, alcohol-related disease or due to a multitude of psychological forces related to the alcohol use.[1] The spectrum of sexual dysfunction encompasses:

Burnett, whose lab has studied penile erection since the early 1990s, continues, "the insight here is tremendous because it speaks to fundamental biological and vascular" mechanisms of diabetes. "This paper gets back to the physiological relevance of hyperglycemia and how it affects erection.  We show here -- using erection as a model -- the vascular damage caused by diabetes and provide insights into vascular disease beyond this dysfunction," he adds.

Similar to the general population (58), in subjects consulting for sexual dysfunction, T deficiency is progressively more prevalent as a function of age (50). In a series of 4,890 subjects consulting our Sexual Medicine and Andrology Unit for sexual dysfunction, one in five (19.6%) and one in three (29.4%) patients have total T below 10.4 and 12 nmol/L, respectively (60). Clinical correlates of T deficiency show different figures according to patient’s age. In fact, we previously demonstrated that in the youngest quartile (17–42 years old), but not in the oldest one (62–88 years old), severity of reported ED and penile blood flow impairment (dynamic peak systolic velocity) were not associated to decreasing testosterone levels (50). It is possible to speculate that, in young individuals, intercourse-related penile erection is such a complex phenomenon that other determinants (i.e., intrapsychic or relational) might mask its androgen regulation and that T deficiency produces greater sexual disturbances in subjects with greater frailty, such as older individuals. However, reported frequency of spontaneous erection and sexual thoughts were significantly decreased as a function of T decline even in younger subjects (50). Moreover, in young individuals low T was associated with a worse metabolic profile, including hypertriglyceridemia and increased waist circumference (50). Accordingly, the prevalence of MetS in the youngest quartile was clearly associated with T deficiency, as it was in the older quartiles (50). Therefore, T deficiency must be accurately verified in all subjects consulting for sexual dysfunction, even in the youngest ones.

Alprostadil self-injection. With this method, you use a fine needle to inject alprostadil (Caverject Impulse, Edex) into the base or side of your penis. In some cases, medications generally used for other conditions are used for penile injections on their own or in combination. Examples include papaverine, alprostadil and phentolamine. Often these combination medications are known as bimix (if two medications are included) or trimix (if three are included).
If this treatment approach fails, it is useful, however frustrating, to start again from the beginning. Reassess the history to ensure that nothing was missed originally. Obtain labs and perform a PCDU. This will give the objective information that you might need in order to make a diagnosis of a known etiology for ED and to provide the patient reassurance that your evaluation has been thorough.
Depression. The profound sadness, emptiness, and hopelessness that characterize depression may also cause ED among younger men. “The biggest effect of depression is on a man’s desire for sexual relations, or libido,” says Drogo Montague, MD, director of the Center for Genitourinary Reconstruction in the Glickman Urological and Kidney Institute at the Cleveland Clinic. “To some extent, depression can affect a man’s ability to maintain an erection. It can be a chicken-and-egg situation. However, reduced libido is a common indicator of depression.”
Penile Injection Medication: This is just what it sounds like. Injected at home directly into the penis, the medication alprostadil produces erection by relaxing certain muscles, increasing blood flow into the penis and restricting outflow. Although some sources report an 80 percent success rate, the therapy has disadvantages, such as risks of infection, pain, and scarring—fibrosis—in the penis, and it may also cause priapism. A popular version of this medication is Upjohn Corporation’s Caverject. The MUSE System, by VIVUS, involves the same medicine (a pellet of alprostadil) applied with an eye-dropper-like applicator, directly into the urethra.