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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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    For reasons mentioned in this post and in How to Ask, we require prior research information when asking questions. See this list of helpful resources. Please help us to help you and edit your question to provide more information on what you have read on this subject, what made you ask this question, and any problems you are having understanding your research. If you found nothing, what did you Google? – LаngLаngС Mar 8 '19 at 17:22
  • I answered your question even though I think @LangLangC is right. I found the answer easily on drugs.com. There are others like rxlist.com and they're equally good. – Carey Gregory Mar 9 '19 at 5:30
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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.

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  • I must have some kind of tolerance to viagra then, because I have used up to 150mg a day, mixed cialis and viagra together, both cialias and viagra with alcohol, and never had any issues...what if I take viagra for sex in the morning and need it in the night at the same day? – Freedo Mar 12 '19 at 4:35
  • @Freedo this is a question for your doctor. – De Novo Mar 12 '19 at 16:24
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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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  • I'd say that 'completely leave' is not entirely correct, mathematically for once but even more important: the first step in metabolising sildafenil produces an active metabolite. That means you'd have an accumulation of 'drugs' in the system, one for a 'desired effect' and a second one 'without desired effect, but with side-effects' (spec; didn't look up the metabolite). AFAIK viagra is really targeted at those aiming for 2x a week, or less, cialis being the better one for more frequent (or more spontaneous) RL action. – LаngLаngС Mar 9 '19 at 8:37
  • @LangLangC 'Completely leave' clearly isn't a mathematically correct statement but rather just a medically accepted standard. The metabolite has the same half-life so no difference there. – Carey Gregory Mar 9 '19 at 12:49
  • @LangLangC Carey is correct about the elimination kinetics here (both sildenafil and its secondary metabolite have a half life of elimination of 4 hrs, while both are on board, and the rule of thumb is 4-5 half lifes for complete clearance). The label recommendations, though, are based on actual observations of tolerability rather than estimates from pharmacokinetics. They're not always as predicted, which is why I made my, perhaps a little snarky post about why any dose is or isn't recommended. – De Novo Mar 9 '19 at 20:31
  • @DeNovo Hm. Isn't, aiming with thumbs, the 'first half-life' of sildafenil 'used' to bind to proteins and produce the UKs? I haven't looked up the actual final fate of all steps but eg ncbi.nlm.nih.gov/pmc/articles/PMC1874255 leads me to believe that substances with side-effect potential may linger in the system a little longer. (2Carey: V from plasma is practically gone, agreed, just a nitpick to better understand it, for me) – LаngLаngС Mar 9 '19 at 20:42
  • @LangLangC I'm going off of the label information. Glancing through the study you linked, it seems to agree, generally. A minor secondary metabolite (7-13%) was observed to have a slightly longer tail of its c/t profile, but it has a pretty low Cmax to begin with. The major secondary metabolite has similar activity to sildenafil, and both probably mediate both intended (treatment) and unintended (side) effects – De Novo Mar 9 '19 at 21:00

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