Michael,   Thanks for your question.  Imdur is a brand name for a  medication called isosorbide mono-nitrate which is a long acting nitrate medication used most commonly in the treatment of coronary artery disease.  It causes dilatation of blood vessels, including the coronary arteries, thereby increasing blood flow to the heart muscle itself.  By dilating blood vessels elsewhere in the body, it reduces the pressure that the heart has to pump against, which helps the heart use less energy and require less oxygen/blood flow from the coronary arteries.    The duration of action of Imdur is 12 hours.    When doctors talk about how long a drug lasts in the system, we usually mention the half-life of a drug.  Half life is the time it takes for 50% of the drug to be completely eliminated from the system.  In two half-lives, 75% of the original dose would be eliminated, and so on.  The half-life of Imdur is 5 hours.  Roughly speaking, in about 24 hours, there might be a couple percent left of the original dose.   I hope this has been helpful.   Martin Cane, M.D.