Phosphodiesterase type V inhibitors (PDE-5 inhibitors) are the most commonly used therapy for erectile dysfunction. These medications work by preventing the breakdown of chemicals that stimulate increased blood flow into the penis. Several different PDE-5 inhibitors are available, which differ slightly in how to use them and their side effects. They appear to be equally effective in the treatment of erectile dysfunction in general, but some individuals may respond to one of these medications more effectively than another.
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.
L-arginine, an amino acid that is naturally present in the body and helps make nitric oxide, supports a successful erection. Nitric oxide is responsible for making the blood vessels relax, which helps sustain an erection for men. A 1999 study, observed the effects of six weeks of high-dose (5 grams/day) orally administered nitric oxide (NO) donor L-arginine on men with organic ED. Thirty-one percent of those who took 5 grams/day of L-arginine experienced significant improvements in sexual function. Burns told Medical Daily, “l-arginine and deer antler velvet” have been the most popular go-to natural treatments for men.
Alprostadil is a potent vasodilator and smooth muscle relaxant identical to the naturally occurring PGE1. PGE1 binds with specific receptors on smooth muscle cells and activates intracellular adenylate cyclase to produce cAMP, which in turn induces tissue relaxation through a second messenger system (96). PGE1 is the only FDA approved form of intracavernosal therapy and is available commercailly as EDEX, or Caverject. Its efficacy was demonstrated in several clinical trials where the rate of responders ranged from 40% to 80% (97,98). The most common adverse event is penile pain, which is not related to the injection of the medication itself. In men with prolonged use the pain is usually self-limited (99).
The great majority of ED cases in diabetic men have a physical cause, such as neuropathy or circulatory problems. In some cases, however, the cause of ED is psychological, including depression, guilt, or anxiety. With a thorough exam, the doctor should be able to determine whether the ED is psychological or physical in nature. If the cause is psychological, your doctor may refer you to a psychiatrist, psychologist, sex therapist, or marital counselor. Do not view such a diagnosis as an insult. Most psychologically-based ED is easily and successfully treated.
NO is produced by the enzyme NO synthase (NOS).  NOS plays many roles, ranging from homeostasis to immune system regulation. To date, 3 subtypes have been identified: nNOS, iNOS, and eNOS, which are produced by the genes NOS1, NOS2, and NOS3, respectively. This nomenclature is derived from the sources of the original isolates: neuronal tissue (nNOS), immunoactivated macrophage cell lines (iNOS), and vascular endothelium (eNOS). The subtypes are not, however, limited to the tissues from which they were first isolated.
These oral medications reversibly inhibit penile-specific PDE5 and enhance the nitric oxide–cGMP pathways of cavernous smooth muscle relaxation; that is, all prevent the breakdown of cGMP by PDE5. It is important to emphasize to patients that these drugs augment the body’s natural erectile mechanisms, therefore the neural and psychoemotional stimuli typically needed for arousal still need to be activated for the drugs to be efficacious.
It is normal for a man to have five to six erections during sleep, especially during rapid eye movement (REM). Their absence may indicate a problem with nerve function or blood supply in the penis. There are two methods for measuring changes in penile rigidity and circumference during nocturnal erection: snap gauge and strain gauge. A significant proportion of men who have no sexual dysfunction nonetheless do not have regular nocturnal erections.
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.
The sympathetic pathway originates from the 11th thoracic to the 2nd lumbar spinal segments and goes via the white rami to enter the sympathetic chain ganglia. Subsequently nerves travel through the lumbar splanchnic to inferior mesenteric and superior hypogastric nerves to the pelvic plexus. The T10 through T12 segments are most often the origin of sympathetic fibers, and the sympathetic chain ganglia that innervate the penis are located in the sacral and caudal ganglia (3).
Because the burning side effect is triggered by alprostadil, the formulations with the least alprostadil — tri-mix and certain versions of bi-mix — could work for men who experience burning with the single-drug formula. But some men might choose alprostadil alone because the multi-drug cocktails can cost more and must be dispensed by a compounding pharmacy — one that is authorized to mix medications on site — which could mean the added hassle of a long drive to pick up the drug.
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Prior to the introduction of PDE5i in 1998, intracavernosal vasoactive medications and penile implant surgery were the mainstays of treatment. Penile implant surgery involves placement of inflatable or malleable rods within the corpora cavernosa to provide rigidity for intercourse. Choice of which implant to place usually depends upon manual dexterity and function of the patient, patient anatomy, physician preference and surgical approach.
Alteration of NO levels is the focus of several approaches to the treatment of ED. Inhibitors of phosphodiesterase, which primarily hydrolyze cGMP type 5, provided the basis for the development of the PDE5 inhibitors. Chen et al administered oral L-arginine and reported subjective improvement in 50 men with ED.  These supplements are readily available commercially. Reported adverse effects include nausea, diarrhea, headache, flushing, numbness, and hypotension.
Stress is your body responding to your environment. And it’s a good thing—in limited doses. When you get stressed out your body makes chemicals like adrenaline that make you stronger, faster, fitter, and even able to think more clearly. Most people call this reaction the “fight-or-flight” response, and it’s a life-saver in dangerous situations. In a very real sense, adrenaline makes you a part-time superhero. The problems happen when your body deals with constant stress.
Cardiovascular diseases: The most common cause of cardiovascular diseases in the United States is atherosclerosis, the narrowing and hardening of arteries that reduces blood flow. Atherosclerosis (a type of vascular disease) typically affects arteries throughout the body; hypertension, high blood cholesterol levels, cigarette smoking, and diabetes mellitus aggravate atherosclerosis. Hardening of the arteries to the penis and pelvic organs, atherosclerosis, causes insufficient blood flow into the penis. There is a close correlation between the severity of atherosclerosis in the coronary arteries and erectile dysfunction. For example, men with more severe coronary artery atherosclerosis (hardening of the arteries in the heart) also tend to have more erectile dysfunction than men with mild or no coronary artery atherosclerosis. Some doctors suggest that men with new onset erectile dysfunction undergo evaluation for silent coronary artery diseases (advanced coronary artery atherosclerosis that has not yet caused angina or heart attacks).
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). 
Alprostadil may also be administered into the urethral opening of the penis. In MUSE (medical urethral system for erection), the man inserts a thin tube the width of a vermicelli noodle into his urethral opening and presses down on a plunger to deliver a tiny pellet containing alprostadil into his penis. The drug takes about 10 minutes to work and the erection lasts about an hour. The main side effect is a sensation of pain and burning in the urethra, which can last about five to 15 minutes.
Another study compared the response of surgically and medically castrated rabbits to vardenafil with that of control rabbits.  Castrated rabbits did not respond to vardenafil, whereas noncastrated rabbits did respond appropriately. This result suggests that a minimum amount of testosterone is necessary for PDE5 inhibitors to produce an erection.
If you have unstable heart disease of any kind, heart failure or unstable, what we call angina, contraindication to using the medications. All right? So if you’re in an unstable medical state, these medications are not a good idea. Now, there are relative issues. If you may be taking a blood pressure medicine or a medicine for your prostate which dilates your blood vessel a little bit– you know, the typical ones are what we call the alpha blockers– you may have an additive effect from the medication. But for the most part, the medicines are incredibly safe.
And surgery comes with its own serious risks, such as developing incontinence or impotence, while radiation can result in urinary problems. — Peter Loftus, WSJ, "Surgery Adds Three Years to Lives of Prostate-Cancer Patients, Study Finds," 12 Dec. 2018 Male-impotence pill Viagra began facing generic competition late last year, while the Lyrica pain treatment is expected to confront generics at the end of this year. — Jonathan D. Rockoff, WSJ, "Pfizer Narrows Guidance on Tougher Pricing, Generic Competition," 30 Oct. 2018 Arkham Horror Third Edition is a classic struggle of agency versus impotence, set in the uncanny world of H.P. Lovecraft’s Cthulhu mythos. — Charlie Theel, Ars Technica, "Arkham Horror Third Edition: The classic Lovecraft adventure returns," 24 Nov. 2018 There’s obviously a lot of anger and frustration here toward the US government at the moment—along with side effects like bottled rage and sometimes a creeping sense of ineptitude or impotence about our ability to do anything about it. — Corey Seymour, Vogue, "Pussy Riot's Nadya Tolokonnikova On Her New Guide To Activism, Raising The Next President Of Russia, And Her "Holy War" On Fairy Tales," 12 Oct. 2018 In exchange for political impotence, they would be mostly left alone and allowed to get rich. — Paul Mozur, New York Times, "Inside China’s Dystopian Dreams: A.I., Shame and Lots of Cameras," 8 July 2018 Their suspicion is compounded by rumors that the polio vaccine causes impotence, death and, ironically, paralysis. — Meher Ahmad, New York Times, "Pakistan Has Just One New Polio Case, but Isn’t Declaring Victory Yet," 20 May 2018 Fighting back, even as an exercise in impotence, did a lot for McCain. — Alex Horton, Washington Post, "John McCain rebelled at the Naval Academy — and as a POW — long before he was a Senate maverick," 3 May 2018 Feelings of desperation and impotence are being felt throughout Central America, where the lawlessness, endemic poverty and levels of gang violence akin to war zones that have driven so many families from their homes show little signs of abating. — Washington Post, "Violence keeps Central Americans coming to US despite Trump," 21 June 2018
Some men report being helped by an oral medication called yohimbine, which comes from the bark of a tree that grows in India and Africa. This drug, which needs to be taken every day, has been reported to help about 20 to 25 percent of the men taking it. A relatively new but widely used oral medication called Viagra requires a careful medical evaluation by your doctor.
Erectile dysfunction (ED), also known as impotence, is the inability to achieve or sustain a hard enough erection for satisfactory completion of sexual activity. Erectile dysfunction is different from other health conditions that interfere with male sexual function, such as lack of sexual desire (decreased libido) and problems with ejaculation release of the fluid from the penis (ejaculatory dysfunction) and orgasm/climax (orgasmic dysfunction), and penile curvature (Peyronie's disease), although these problems may also be present. ED affects about 50% of men age 40 and over. This article focuses on the evaluation and treatment of erectile dysfunction.
When it comes to boosting sexual performance, many men will walk all over God’s green earth looking for ways to maintain a good sex life. Luckily men, all you have to do is walk — not run — 2 miles a day. This, along with other healthier lifestyle interventions can help obese men reduce their risk of ED, or even “reverse” current impotence, according to a 2005 study. This comes of importance, since maintaining a trim waistline is a good defense for ED, as men with a 42-inch waist are 50 percent more likely to have ED than those with a 32-inch waist. Getting to a healthy weight and maintaining it is a good strategy for preventing and treating ED.
Then you have to be able to make the right diagnosis. What is the basis for their erectile dysfunction? Is it psychogenic? Is it some sort of neurological or blood vessel or hormonal issue? So you have to make a diagnosis. You have to be able to make an assessment. And then only after those things are done, then you start to think about medications.
The laboratory results should be discussed with the patient and, if possible, with his sexual partner. This educational process allows a review of the basic aspects of the anatomy and physiology of the sexual response and an explanation of the possible etiology and associated risk factors (eg, smoking and the use of various medications). Treatment options and their benefits and risks should be discussed. This type of dialogue allows the patient and physician to cooperate in developing an optimal management strategy.
In comparison, 37% of men who had received external radiotherapy as their primary therapy reported the ability to attain functional erections suitable for intercourse, along with 43% of men who had received brachytherapy as primary treatment. Pretreatment sexual health-related quality of life score, age, serum prostate-specific antigen (PSA) level, race or ethnicity, body mass index, and intended treatment details were associated with functional erections 2 years after treatment.