Injection therapy involves injecting a substance into the penis to enhance blood flow and cause an erection. The Food and Drug Administration (FDA) approved a drug called alprostadil (Caverject) for this purpose in July of 1995. Alprostadil relaxes smooth muscle tissue to enhance blood flow into the penis. It must be injected shortly before intercourse. Another, similar drug that is sometimes used is papaverine—not yet been approved by the FDA for this use. Either drug may sometimes cause painful erections or priapism (uncomfortable, prolonged erections) that must be treated with a shot of epinephrine.
Treatment. prostatitis or another acute infection affecting the genitalia can cause temporary impotence that clears up in response to antibiotics. The smooth muscle relaxant sildenafil (viagra) was introduced in 1998 as a treatment for organic impotence. Administration of testosterone may be indicated if low levels of this hormone are found in a blood sample. If impotence is organic and does not respond to other therapies, a penile prosthesis can be implanted; this is usually done surgically by a urologist. Other therapies include the use of vacuum tumescence devices and penile injection of pharmacologic agents that cause dilation.
The dorsal artery provides for engorgement of the glans during erection, whereas the bulbourethral artery supplies the bulb and the corpus spongiosum. The cavernous artery effects tumescence of the corpus cavernosum and thus is principally responsible for erection. The cavernous artery gives off many helicine arteries, which supply the trabecular erectile tissue and the sinusoids. These helicine arteries are contracted and tortuous in the flaccid state and become dilated and straight during erection. 
An analysis of 14 studies involving more than 90,000 patients with ED confirmed the relation between ED and an increased risk of cardiovascular events and mortality.  Compared with patients without ED, those with ED had a 44% increased risk of cardiovascular events, a 25% increased risk of all-cause mortality, a 62% increased risk of MI, and a 39% increased risk of cerebrovascular events. Treatment of ED, either through lifestyle interventions or by pharmacologic means, may improve prognosis and reduce risk.
Given the high risk of priapism during escalation of therapy for intracorporeal injection, it is recommended that the drugs be administered in a supervised office visit initially and that the patient be given a well-articulated plan for treatment of priapism if it occurs. Escalation guidelines for alprostadil alone vary, but a general guideline is to start at 2.5 mcg and increase by 2.5 mcg to a dose of 5 mcg and then in increments of 5 mcg to 10 mcg until an erection sufficient for penetration, not lasting more than 1 hour, is achieved. If there is no response to the initial 2.5-mcg dose, escalation dosing can be slightly more liberal.34 A European prospective trial of PGE1 alone found 91% of the 54 patients completing the 4 years of the study reported good or better tolerability and satisfaction with therapy.35
In rare cases, the drug Viagra ® can cause blue-green shading to vision that lasts for a short time. In rare cases, the drug Cialis® can cause or increase back pain or aching muscles in the back. In most cases, the side effects are linked to PDE5 inhibitor effects on other tissues in the body, meaning they are working to increase blood flow to your penis and at the same time impacting other vascular tissues in your body. These are not ‘allergic reactions'.