A drug formulation that releases the active ingredient at a steady rate. This can be done by a variety of technologies, including liposomes, subcutaneous injections of a polymer matrix, and transdermal patches. Controlled release of a drug can increase patient comfort and compliance by reducing the number of injections or oral doses that must be taken, as well as avoiding the "peak and valley" profile of drug concentration in the blood that is usually seen after a dose of conventional formulations.