Describes a study in which patients participate in each arm of the study in random order. For example, a patient in a trial testing a new drug against a placebo might be assigned to begin with the placebo. After a specified amount of time, that patient would then be switched to the actual drug being tested. This allows each patient to act as their own control and eliminates problems with ensuring that the patients in different arms of a study are similar enough to compare the results.
However, crossover studies have some drawbacks. They cannot be used to compare two curative approaches to each other (like antibiotics, for instance). They also often require a “washout” period between the treatments studied, to minimize the effect of interactions between the drugs.
Crossover studies are only appropriate for short-term studies, meaning they are inappropriate to study different disease-modifying therapiesfor multiple sclerosis (MS), which require longer periods to evaluate effectiveness. However, they are useful to study treatments to reduce symptoms of MS.