The role of the endothelium in ED has been noted for a number of years and the overlapping of ED and other conditions, especially coronary heart disease, CVD, affecting endothelial function/dysfunction, is clearly present. The endothelial cell is now known to affect vascular tone and impact the process of atherosclerosis, and impacting ED, CVD and peripheral vascular disease.16
Alprostadil, a synthetic PGE1, is the most commonly used single drug for injections into the penis as a treatment for ED. It works well in the majority of men who try it. In one study of 683 men with ED, 94% reported having erections suitable for penetration after PGE1 injections. When PGE1 is used in combination with papaverine and Regitine, the mixture is called Trimix, which has roughly twice the effectiveness of alprostadil alone. However, Trimix is quite expensive and is usually not covered by insurance, while PGE1 is often a covered benefit in most insurance medication plans. The main side effects are pain from the medication (not from the injection), priapism (persistent or abnormally prolonged erection), and scarring at the site of the injection. It is recommended that one alternate the side of the penis being injected to help decrease the risk of penile scarring. Many men are uncomfortable with penile injection therapy even though the injection itself is painless. The injection cannot be done more often than every other day. Men on anticoagulant medications (blood thinners) must be careful with injection therapy.
However, a review of a United Kingdom medical record database found no evidence that the use of 5-alpha reductase inhibitors independently increase the risk for ED. In 71,849 men with benign prostatic hyperplasia (BPH), the risk of ED was not increased with the use of finasteride or dutasteride only (odds ratio [OR] 0.94), or a 5-alpha reductase inhibitor plus an alpha blocker (OR 0.92) compared with an alpha blocker only. In addition, the risk of ED was not increase in 12 346 men prescribed finasteride 1 mg for alopecia, compared with unexposed men with alopecia (OR 0.95). The risk of ED did increase with longer duration of BPH, regardless of drug exposure. 
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These are not currently approved by the FDA for ED management, but they may be offered through research studies (clinical trials). Patients who are interested should discuss the risks and benefits (informed consent) of each, as well as costs before starting any clinical trials. Most therapies not approved by the FDA are not covered by government or private insurance benefits.
En español | If you watch TV or read magazines, you could easily conclude that men seeking treatment for erectile dysfunction (ED) have but one option: pills. The three dominant brands — Cialis, Levitra and Viagra — are locked in an expensive battle for men’s allegiances, and have, through their suggestive advertising, triggered both satire and controversy in the 14 years since the Food and Drug Administration approved Viagra.
Normal erectile function depends on the release of NO and endothelial-dependent vasodilation of the penile arteries. The ‘artery size’ hypothesis, first described by Dr Montorsi, offers an explanation why men are more likely to develop ED before a myocardial infacrtion. It is believed that atherosclerosis affects all vascular beds equally but smaller arteries are more likely to become occluded than larger arteries.31 32 The penile arteries are 1–2 mm while the coronary arteries are 3–4 mm. Thus, the same degree of endothelial dysfunction and atherosclerosis is more likely to occlude blood flow in the penile arteries compared with the coronary arteries. The penile arteries therefore serve as a sensitive indicator for subsequent CVD. This theory is supported by the fact that ED occurs approximately 3 years prior to cardiac symptoms in virtually all patients with chronic coronary syndrome whereas patients with acute coronary syndrome have a much lower prevalence of sexual dysfunction.32
I would think the 25% is just an average. Some men never have a problem with erections. Some healthy men in their 60's never have a problem with their erections -- no Viagra needed at all, thank you. And, no, it is NOT completely associated with age. In fact, many men in their 40's and even younger, are already dealing with ED, especially if they smoke, are obese, don't exercise, or are addicted to porn, among other things. In fact, even men in their 20's and 30's can have these problems.
Erectile dysfunation medications may not be suitable for you if you have been told not to have sex, not to engage in sexual activity/actions that widen your blood vessels, have low blood pressure (hypotension), have recently had a stroke, have unstable angina, have had a heart attack or have a history of non-arteritic anterior ischaemic optic neuropathy. Men with cardiovascular disease/anatomical issue with their penis/priapism/taking long-lasting alpha-blockers should use with caution and discuss with a doctor first. Do not mix with drugs/medications containing nitrates.
Trauma to the pelvic blood vessels or nerves can also lead result in ED. Bicycle riding for long periods has been implicated as an etiologic factor; direct compression of the perineum by the bicycle seat may cause vascular and nerve injury.  On the other hand, bicycling for less than 3 hours per week may be somewhat protective against ED.  Some of the newer bicycle seats have been designed to diminish pressure on the perineum. [37, 38]
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'.