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Medicines
Four main substances:
There are four substances that have medically proven effect. These are: Sildenafil, Vardenafil, Tadalafil and Avanafil. These medicines are sold as prescription medications under brand names such as: Viagra (sildenafil), Levitra (vardenafil), Staxyn (vardenafil), Cialis (tadalafil) , Adcirca (tadalafil), and Stendra (avanafil).
How do they work?
These medicines all work in a the same way. It can be complicated to explain how, but here is a short version:
As described under the page "Treatments", the main molecule for successful erections is Nitric Oxide (NO). Without good levels of NO - no good erection!
Upon arousal NO starts a process that raises the levels of cGMP (cyclic guanosine monophosphate) in the penis. cGMP is needed to get an erection. However, there is a also "bad" enzyme there called PDE5 (phosphodiesterase 5). PDE5 breaks down cGMP by binding to it and if that happens too fast, there is no good erection.
Enter onto the stage the ED medicines:
To the "bad" enzyme PDE5, they all look like cGMP. Therefore, PDE5 is tricked into binding to them instead of cGMP.
Result: More free cGMP leads to better erection. Case solved! :-)
What are the differences?
Don't get fooled by all the brand names. The same amount of Sildenafil has exactly the same effects, no matter if it is called Viagra or Kamagra or Revatio.
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Here is an overview of the main differences:
Vardenafil
Activation Period: 20-60 minutes
Strong effect for: 5 hours
Dosages: 5-20 mg
Effect down by food: Yes
Sildenafil
Activation Period: 20-60 minutes
Strong effect for: 4 hours
Dosages: 25-100 mg
Effect down by food: Not much
Tadalafil
Activation Period: 1-2 hours
Strong effect for: 17.5 hours
Dosages: 2.5-20 mg
Effect down by food: Yes
Avanafil
Activation Period: 20-60 minutes
Strong effect for: 5 hours
Dosages: 50-200 mg
Effect down by food: Not much