The first time-release medication was probably just a sugarcoated pill.
The coating did not dissolve and release the bitter medicine until it was safely past the mouth.
Many time-release medications, like Contac, introduced in 1961, are based on that fairly simple idea: small amounts of the medication are coated with varying thicknesses of soluble material.
The thicker the coating, the longer it takes to dissolve in the body.
If exactly the right number of pellets of exactly the right thicknesses is in each capsule, the dose is released gradually over a fairly predictable period.
However, the release actually takes place in tiny spurts, not continuously.
Some drugs that can be absorbed through the skin, like nitroglycerine, can be put in patches that release the drug both slowly and constantly.
One system to deliver scopolamine to fight seasickness uses a disk with an impermeable backing, an ultra thin reservoir holding the drug, a membrane with microscopic pores to control the drug release and an adhesive surface to hold it on the skin.
Some drugs, like hormone implants used as birth control, use a similar system in pellets implanted in fatty tissue under the skin.
A recently patented timed delivery system for a drug that does not dissolve in water uses a pair of separated compartments.
One compartment contains a polymer that draws water into the pill through a semipermeable membrane.
The inflow of water increases pressure within the pill, pushing medication out of the other compartment through a tiny hole.