A parallel study is a type of clinical study in which two or more groups of participants receive different interventions. Participants are assigned to one of the treatment arms at the beginning of the trial and continue in that arm throughout the length of the trial. Assignment to a group usually is randomized. Study participants are only exposed to the treatment that is assigned to the particular study arm they are enrolled in.
For example, a two-arm parallel assignment involves two groups of participants. One group receives drug A, and the other group receives drug B. So during the trial, participants in one group receive drug A “in parallel” to participants in the other group, who receive drug B.