A trial arm is a group of participants that receives the same interventions, or no intervention, according to the study protocol. Many randomised trials have two arms, but some may have three or even more. This is decided before the trial begins.
Trials with several arms (multi-arm) allow more than one treatment to be tested at once, and can reduce the costs and time needed during clinical development. Multi-arm, multi-stage (MAMS) trials take this idea a step further and allow the recruitment of participants in a particular arm to be stopped partway through if that treatment is not producing satisfactory results. MAMS can also allow for new treatments to be added to the trial as they become ready for testing.