Hey there! Imuran works by continued use. Its half life is really short- After about a day, all the Imuran from the previous day would be metabolized. From personal experience, I always get the nausea and stomach related side effects if I miss my Imuran for more than a day, more than a week and my blood tests start to go crazy. Haha don't do what I do. ;) just start taking it as soon as possible, and don't try to make up for your missed doses.