This article is from the Asthma FAQ, by Patricia Wrean and Marie Goldenberg firstname.lastname@example.org with numerous contributions by others.
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.