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The expression pattern of PDE11 has become the source of some controversy recently because of the drug tadalafil. Of the three major PDE5 inhibitors commonly used to treat erectile dysfunction (tadalafil, sildenafil, and vardenafil), only tadalafil has been found to have a relatively high affinity for PDE11 with a reported IC50 of ?73 nM (Tables 4 and 5) (Card et al., 2004; Weeks et al., 2005). Therefore, some scientists speculate that tadalafil will be more likely to have nonspecific side effects than sildenafil or vardenafil because of inhibitory effects on PDE11. Given that tadalafil inhibits PDE5 with substantially higher affinity (IC50 ?10 nM), our lack of information about physiological roles for PDE11, and the very similar safety profiles for all three drugs, it will probably take long-term trials to determine the validity of these speculations. How to buy Viagra and get free samples viagra no prescription buy Viagra 50mg here brand viagra online without prescription Overview and symptoms viagra at canadian pharmacy canadian pharmacy viagra for sale online gClinica Urologica 'Federico II' Universita degli Studi di Napoli, Italia While oral medicines improve the response to sexual stimulation, they do not trigger an automatic erection as injections do. The recommended dose for Viagra is 50 mg, and the physician may adjust this dose to 100 mg or 25 mg, depending on the patient. The recommended dose for either Levitra or Cialis is 10 mg, and the physician may adjust this dose to 20 mg if 10 mg is insufficient. A lower dose of 5 mg is available for patients who take other medicines or have conditions that may decrease the body's ability to use the drug. Levitra is also available in a 2.5 mg dose. Despite considerable recent experimentation in animal models and human volunteers, information on the central pathways of erection remains cursory at best.18 It is known that androgens play a predominantly modulating role by their effect on libido and sexual behaviour. Testosterone enhances sexual interest and the frequency of sexual acts; it increases the frequency of nocturnal erections but does not effect reflexogenic or psychogenic erections.19