Hi Francis,     Studies have shown that Xanax (alprazolam) immediate release tablets show peak concentrations of the drug in blood plasma after about one to two hours. The average "half-life" of Xanax, which means the time it takes the blood to eliminate half of the drug is 11.2 hours (with a range of 6.3 to 26.9 hours). Xanax XR is an extended release version of the drug. It has a slower absorbtion rate, in that the body maintains a constant concentration of the drug 5 to 10 hours after you take it.  However, studies are showing that elimination of the XR version is about the same as the immediate release. You didn't say why you asked this question, but if you have taken Xanax recently and are concerned about its effects on your body I urge you to contact your doctor as soon as possible and express your concerns.   Regards, Dr. Jennifer Fee www.thestressmasters.com