2.4 What kinds of tablets are there?
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- 2.4.1 Why do I need a blood test when taking theophylline?
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- 2.4.2 Why are combination pills not commonly prescribed?
2.4 What kinds of tablets are there?
- CR
- controlled release. This means that the drug has a
constant rate of release.
- DR
- delayed release. This generally refers to enteric-
coated tablets which are designed to release the drug
in the intestine where the pH is in the alkaline range.
- ER
- extended release. Dosage forms which are designed to
release the drug over an extended period of time, such
as implants which release the drug over a period of
months or years.
- SA
- sustained action. Used interchangeably with CR
(above), except that SA usually refers to the
pharmacologic action while CR refers to the drug
release process.
- TD
- time delayed. This is slightly different from DR in
that the drug release is designed to occur after a
certain period of time, such as pellets coated to a
certain thickness, multi-layered tablets, tablets
within a capsule, or double-compressed tablets.
Contributed by:
Susan Graham / sgraham@hpb.hwc.ca
Theophylline is commonly used as a third-line agent in the
management of asthma, after beta-agonists and anti-inflammatories.
Unfortunately, its therapeutic level is quite close to its toxic
level. This means that the dose that the asthmatic needs to get
the full benefit of the drug is not very much lower than the dose
which causes side effects which range from unpleasant to
dangerous. This would not be such a problem if there weren't
such large variations in the rate at which people metabolize
theophylline. Apparently, if a group of people are given
the same dose of theophylline, the concentration of the
drug in their bloodstreams may vary by up to a factor of
seven. Therefore, the best way to monitor that the asthmatic
is receiving the optimal amount of theophylline is to take
a blood level concentration.
The combination drugs such as Tedral and Marax commonly
contain theophylline, ephedrine, and some form of sedative
such as phenobarbital. These combination pills are no longer
commonly prescribed because the amount of theophylline in
the pill cannot be varied with respect to the other drugs.
Since there is great variation in the rate at which an
individual metabolizes theophylline, it is now considered
better to take theophylline separately, for better adjustment
of theophylline levels. In fact, Tedral is no longer
manufactured by Parke-Davis in the U.S.
Also, ephedrine is no longer considered the bronchodilator
of choice. From Drs. Haas, The
Essential Asthma Book,
"ephedrine initiates the release of catecholamines -- including
adrenaline -- that are already stored in the body. This is
its biggest drawback. Its effects depend on the availability
of catecholamine in the body at the time it is given, and
these concentrations vary." Since much better bronchodilators
are now available, ephedrine is no longer commonly prescribed.
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