The term therapeutic window refers to the range of
doses of a drug that are actually effective in treating a
particular disease. Doses below the therapeutic window are too weak to
have any effect; doses above the window cause unacceptable
side-effects. Doctors will sometimes say that "the
therapeutic window is closed," meaning that there exists no dose that will
give benefits without harmful side-effects--you're in a damned-if-you-do,
damned-if-you-don't situation.
Sometimes the therapeutic window is closed from the start. In
epilepsy, for example, patients are often put on anticonvulsant drugs.
In severe cases, a dose that's high enough to stop the seizures
will also make the patient stuporous. In other cases, the
therapeutic window closes as the patient develops a tolerance to the
drug. In Parkinson's disease, patients are given a dopamine agonist
to treat their tremors, rigidity, and bradykinesia. For a
while, it works, and they can move just like healthy people. After some
time, though, this dose causes dyskinesia and patients can't control
their movements. Eventually, the drugs will either do nothing or cause
involuntary movements, depending on the dose you give.
| 
