Many drugs that are active in vitro turn out to be inactive in vivo. Before any medicinal chemistry project is started, it is important to establish what testing procedures are going to be used to test whether compounds have the desired activity and how strongly they interact with the desired target. In vivo tests are restricted to potentially useful compounds and are used to test whether active compounds in vitro are active in vivo. In vivo tests will also determine whether a drug has the physiological effect desired. Sometimes, compounds are found which are active in vivo, but are inactive in vitro. This is usually because the compounds involved are acting as prodrugs. In other words, they are not active themselves, but are metabolized in the body to the active compound.