Why you can't take viagra more than once a day?
It's said Viagra effects only last around 3-4 hours, and it stay in the body up to 8 hours.
So why is the recommendation to not take more than 100mg/daily comes from?
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Why is the recommendation to not take more than 100mg/daily?
Because that is what the clinical data show. If you take more than 100mg/daily you are at a higher than acceptable risk of undesirable effects.
As to where this information comes from...
Where does the recommendation for (anything related to dose for any medication approved by a modern regulatory authority) come from?
Initial estimates for dosing are based on preclinical studies of pharmacokinetic and toxicological studies in animals. Final recommendations are based on a series of three general types of clinical trials in human subjects.
Phase I clinical trials establish pharmacokinetic properties, safety and tolerability of a medication, using a range of doses. These data are used to estimate likely effective and safe doses in volunteers with the disease of interest. Sometimes this is described as Phase 0 and Phase I, or Phase Ia and Phase Ib, but traditionally, this is all considered Phase I.
Phase II clinical trials are primarily aimed at efficacy (whether the drug provides a benefit) in volunteers with the disease of interest. Though efficacy is the main concern here, these trials do continue to monitor safety and tolerability, and can collect additional pharmacokinetic data as well. Data from Phase II clinical trials is used to design Phase III clinical trials. The doses tested in Phase III may be adjusted from what was predicted by Phase I.
Phase III trials provide more information about both safety and efficacy, using a much larger number of subjects, typically in multicenter studies. Here again, the research subjects are volunteers with the disease of interest. Once Phase III trials are complete, an application is submitted to the regulatory authority.
The label information, including dose recommendations, is approved by the regulatory authority based on the aggregate data from these studies.
Goodman & Gilman's The Pharmacological Basis of Therapeutics discusses this process in Chapter 1. You can also read about it on Wikipedia, but this is not the best Wikipedia article I've read.
Viagra (sildenafil ) has a half-life of about 4 hours. It takes five half-lives for a drug to completely leave your system, so the time it takes for Viagra to leave your system is more like 20 hours, not eight. That's probably the reason for the once daily recommendation.
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