Reoes anyone know why the label on Viagra says not to take with grapefruit juice? eom

Inside our intestines, special enzymes break down drugs before they reach the bloodstream. The flavonoid compounds in grapefruit block the effects of these enzymes. When that happens, it's easier for certain drugs to pass from your intestines to your bloodstream. So levels of those drugs rise faster and higher than normal. As an example, the amount of Lipitor absorbed into the blood stream is 20 percent to 50 percent higher if the drug is taken with grapefruit juice. That means if you take a 10-milligram (mg) dose of Lipitor, you could get the equivalent of 12 to 15 mg of Lipitor if you took if with grapefruit juice. High levels can increase the risk of side effects, such as muscle damage. However, these types of interactions seem to be rare.

thereby increasing both the concentration of specific drugs in the blood stream (increases the amount) and prolongs their presence in the blood stream....  The specific family of enzymes involved include some in the P450 family.  Also some of the same components are found in apple juice as well! lol  and probably grape juice...

It is also know that while the concentration and persistance of some drugs are increased, others are diminished - such as fexofenadine and digoxin may be lowered by inhibiting drug absorption catalyzed by the organic anion transporting polypeptide.

In the case of Viagra (Sildinafil) the cytochrome P450 enzyme affected is  intestinal CYP3A4.  Action of this drug increases the quantity over time of Viagra by 25% and prolongs the time to maximum blood concentration by about 50%....  In short, Grapefruit Juice increases the effective dose of Viagra but delays absorption....  Probably not a biggie - but play it safe.

Would taking your viagra with coca cola instead of water increase or decrease it's potency ?
                          Thanks

Urban legend has it that just like drinking Cola with the candy Pop Rocks, it will cause your insides to explode.

at least in a limited # of studies, with a number of xenobiotics, increases absorption and blood concentration.... soooooo yea, there would be a difference - similar to grapefruit juice.... Go figure.