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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'.
When you become aroused, your brain sends chemical messages to the blood vessels in the penis, causing them to dilate or open, allowing blood to flow into the penis. As the pressure builds, the blood becomes trapped in the corpora cavernosa, keeping the penis erect. If blood flow to the penis is insufficient or if it fails to stay inside the penis, it can lead to erectile dysfunction.
Erectile dysfunction can cause strain on a couple. Many times, men will avoid sexual situations due to the emotional pain associated with ED, causing their partner to feel rejected or inadequate. It is important to communicate openly with your partner. Some couples consider seeking treatment for ED together, while other men prefer to seek treatment without their partner's knowledge. A lack of communication is the primary barrier for seeking treatment and can prolong the suffering. The loss of erectile capacity can have a profound effect on a man. The good news is that ED can usually be treated safely and effectively.
The chemical synthesis of testosterone from cholesterol was achieved in August that year by Butenandt and Hanisch. Only a week later, the Ciba group in Zurich, Leopold Ruzicka (1887–1976) and A. Wettstein, published their synthesis of testosterone. These independent partial syntheses of testosterone from a cholesterol base earned both Butenandt and Ruzicka the joint 1939 Nobel Prize in Chemistry. Testosterone was identified as 17β-hydroxyandrost-4-en-3-one (C19H28O2), a solid polycyclic alcohol with a hydroxyl group at the 17th carbon atom. This also made it obvious that additional modifications on the synthesized testosterone could be made, i.e., esterification and alkylation.
Hormone deficiency or hypogonadism, whether primary or secondary, has been thought to impact erectile function. Approximately a third of men in the European Male Aging Study demonstrated low testosterone, suggesting that hypogonadism is overrepresented among men with ED.11 Hormone deficiency, however, is less frequently the cause of ED than diabetes or vascular disease. Many entities with a strong relationship to ED also diminish bioavailable testosterone, including obesity, diabetes, and opioid use. Other hormones involved in testosterone metabolism or availability, like thyroid stimulating hormone and gonadotropins, also may impact erectile quality, presumably through regulating bioavailable testosterone. Understanding the relationship between testosterone and ED has been impaired by a lack of standardized measurement of this hormone and the cyclic nature of its release and consumption.
The partial synthesis in the 1930s of abundant, potent testosterone esters permitted the characterization of the hormone's effects, so that Kochakian and Murlin (1936) were able to show that testosterone raised nitrogen retention (a mechanism central to anabolism) in the dog, after which Allan Kenyon's group was able to demonstrate both anabolic and androgenic effects of testosterone propionate in eunuchoidal men, boys, and women. The period of the early 1930s to the 1950s has been called "The Golden Age of Steroid Chemistry", and work during this period progressed quickly. Research in this golden age proved that this newly synthesized compound—testosterone—or rather family of compounds (for many derivatives were developed from 1940 to 1960), was a potent multiplier of muscle, strength, and well-being.
In a prospective study from the Prostate Cancer Prevention Trial database, Thompson et al reported that men presenting with ED had a significantly higher chance of developing a cardiovascular event over a 7-year follow-up period.  The hazard ratio was 1.45, which is in the range of risk associated with current smoking or a family history of MI.
One study examined the role of testosterone supplementation in hypogonadal men with ED. These men were considered nonresponders to sildenafil, and their erections were monitored by assessing nocturnal penile tumescence (NPT). After these men were given testosterone transdermally for 6 months, the number of NPTs increased, as did the maximum rigidity with sildenafil.  This study suggests that a certain level of testosterone may be necessary for PDE5 inhibitors to function properly.
Type 2 diabetes is an important condition in terms of morbidity and mortality, and the prevalence is increasing in the developed and developing world. The prevalence also increases with age. Insulin resistance is a primary pathological feature of type 2 diabetes and predates the onset of diabetes by many years, during which time raised serum insulin levels compensate and maintain normoglycemia. Insulin resistance and/or impaired glucose tolerance are also part of the metabolic syndrome which also comprises an abnormal serum lipid profile, central obesity and hypertension. The metabolic syndrome can be considered to be a pre-diabetic condition and is itself linked to cardiovascular mortality. Table 1 shows the three commonly used definitions of the metabolic syndrome as per WHO, NCEPIII and IDF respectively (WHO 1999; NCEPIII 2001; Zimmet et al 2005).