crossover

Definition of crossover study
National Cancer Institute

crossover study

(KROS-oh-ver STUH-dee)
A type of clinical trial in which all participants receive the same two or more treatments, but the order in which they receive them depends on the group to which they are randomly assigned. For example, one group is randomly assigned to receive drug A followed by drug B. The other group receives drug B followed by drug A. There is usually a rest period between treatments.

Source: NCI Dictionary of Cancer Terms