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Crossover study design


The administration of two or more experimental therapies one after the other in a specified or random order to the same group of patients.

There are some important issues with cross-over designs. Two in particular often crop up.

First is the issue of order effects, in which the order in which treatments are administered may affect the outcome. An example might be a drug with many adverse events 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. In practice carry-over can be dealt with by use of a wash-out period between treatments, or by making observations sufficiently later after the start of a treatment period that any carry-over effect is minimised.