Many of the medications we take for our disorders come in immediate release or extended release forms.
Immediate release, as the name implies, dissolves immediately within our stomachs and is taken up into the bloodstream. This can cause side effects, it is also taken up quickly and then degraded quickly. So the concentration of the medicine does not stay constant in our bodies, we get a quick spike and the drug is quickly degraded. It is actually better to have a constant concentration as opposed to this increase and decrease. Lithium is one drug that is known to cause major gastrointestinal side effects if taken in large doses in an immediate release form and can be quite corrosive to the stomach.
Extended release, also known as Controlled release, has a coating that allows it to dissolve slowly either in the stomach or small intestine (enteric coated). These medicines are then slowly dissolved, therefore slowly absorbed into the bloodstream. This not only cuts down on local side effects such as upset stomach, but general side effects are also reduced. The dose of the medicine stays more constant over a 12 hour period. This is much more desirable.