Complex sentence: The use of theophylline is complicated by its interaction with various drugs and by the fact that it has a narrow therapeutic window (i.e., low therapeutic index), its use must be monitored by direct measurement of serum theophylline levels to avoid toxicity.
Split into simpler sentences: The use of theophylline is complicated by its interaction with various drugs and by the fact that it has a narrow therapeutic window. Its use must be monitored by direct measurement of serum theophylline levels to avoid toxicity.