Viagraīecause sexual activity can stress the heart, men who have heart problems should check with their physician to see if sexual activity is recommended. In 2004, Viagra had about 66% of the market share for ED drugs compared to 19% for Levitra and 14% for Cialis. Sales of the three newer ED drugs reached $3.4 billion in 2004, according to the research firm IMS Health. Before Viagra's approval in 1998, drug treatment of erectile dysfunction was limited to alprostadil (prostaglandin ), either injected into the penis or inserted as a pellet into the urethra. ![]() Men should not have sex more than once every 24 hours after using these drugs. Each is effective for roughly four hours. Sildenafil or vardenafil should be taken about an hour before sex. These pills do not cause an erection by themselves. This chemical relaxes muscles in the penis to allow more blood to flow in. All three pills work by enhancing the effects of nitric oxide. Several studies have indicated that their success rate in diabetic men may be slightly lower, averaging around 60-65%. ![]() They help about three-fourths of all men who try them in the general population. ![]() There are three prescription medications available in the United States to treat the physical causes of ED: sildenafil citrate (Viagra), vardenafil (Levitra), and tadalafil (Cialis).
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