Drug Half-Life: Why Viagra Works for 4 Hours But Cialis Works for 24 Hours

www.acsh.org
3 min read
fairly easy
If you don't learn anything from this pharmacology lesson, at least remember this one thing: Call your doctor if your erection lasts longer than 4 hours. Seriously.
Sometimes, guys need a little help in the bedroom, and one option is to use a drug like Viagra or Cialis. The former works for about four hours, while the latter can work up to 24 hours or even longer. Why? Aren't they essentially the same thing?

Yes and no.

Yes, they do the same thing (schwing!) using the same mechanism (i.e., inhibition of an enzyme called PDE5). But no, they are actually quite different. Though nowhere nearly as grueling as my colleague Dr. Josh Bloom's Dreaded Chemistry Lesson from HellTM, this article gets a little wonky.

Pharmacology 101

A drug's effect on the body is based on a combination of things, such as the physical nature of the drug itself (which involves a combination of molecular composition, shape, size, and charge), the body's ability to absorb the drug (e.g., how well the gastrointestinal tract absorbs it), the cellular targets that are available for the drug to bind to, and the body's ability to eliminate the drug. Taken together, such characteristics help determine the proper dose. The doses for sildenafil (Viagra) and tadalafil (Cialis) are shown below:

Source: Pan Pantziarka et al.,…
Read full article