6-15. MODES OF OPERATION
The initial dose of sustained-release medication provides sufficient drug at the
time it is administered to produce the desired therapeutic effect. Thereafter, as the drug
is removed from the body, more drugs, the sustained-release, must be supplied to keep
a constant level in the blood. There are various means of achieving this.
a. Capsules. The original medication in this category is a capsule containing a
number of small pellets. A full and immediate dose is provided to the bloodstream from
these capsules by uncoated pellets that dissolve quickly. Other pellets, coated with
varying thicknesses of a slowly soluble aterial, supply the bloodstream with enough drug
to keep the concentration at the desired level. In addition, ion exchange resins have
been used in some sustained-release capsules to release drugs at the proper intervals.
b. Tablets. In addition to capsules, some tablets provide sustained release.
(1) Granules. Some sustained-release tablets are made by compressing
granules with varying coats into a tablet. The coats dissolve at different times, much
like those in the capsules discussed above.
(2) Slowly soluble core. Some tablets are manufactured with an outer
coating that dissolves quickly to give the initial dose. The core of this tablet, consisting
of a slowly soluble base containing additional drug provides the sustained release. It is
possible to achieve the same effect with layered tablets, which are often flat and
cylindrical to give a constant sustained dose.
(3) Insoluble matrix. This tablet is an insoluble network of channels
containing the drug. Enough drug is immediately extracted from the outer channels to
provide the initial dose. Thereafter, the drug is extracted from the inner channels of the
tablet less rapidly, producing the sustained release.
(4) Ion exchange resins. Ion exchange resins are sometimes used in
tablets to provide sustained release.
(5) Repeat-action tablets. Repeat-action tablets are not really sustained-
release tablets. They consist of an outer layer that dissolves quickly and provides the
initial dose. Within is another tablet with a special coating that dissolves in about four
hours and releases another full dosage. It is useful in avoiding the necessity of waking
in the middle of the night to take a second tablet.