These studies are often done to improve the symptoms of patients with chronic conditions ; for curative treatments or rapidly changing conditions, cross-over trials may be infeasible or unethical.
Crossover studies often have two problems:
First is the issue of "order" effects, because it is possible that the order in which treatments are administered may affect the outcome. An example might be a drug with many adverse effects given first, making patients taking a second, less harmful medicine, more sensitive to any adverse effect.
Second is the issue of "carry-over" between treatments, which confounds the estimates of the treatment effects. In practice, "carry-over" effects can be avoided with a sufficiently long "wash-out" period between treatments. However, the planning for sufficiently long wash-out periods does require expert knowledge of the dynamics of the treatment. which often is unknown, of course.
