In clinical trials, one group of participants (an arm of the trial) receives one treatment (drug) while another arm receives a different treatment or placebo. Most randomised trials have two arms, but some have three or even more

See also:
Clinical trial
Placebo
Placebo-controlled studyBaseline
Bias
Blind
Brief Summary
Baseline
Blind (clinical treatment)
Clinical Trial  Enrolement
Clinical
Clinical Trial
Cohort
Control Group
Controlled Trial
Diagnostic Trials
Dichotomous Question
Dose-ranging Trial
Double blind Trial
Drop Out
Drug-drug interaction
Effectiveness
Efficacy
Eligibility Criteria
Endpoint
Experimental Drug
Informed Consent
International Clinical Trial Registry Platform (ICTRP)
Methodology
Open label Trial
Phase I Trials
Phase II Trials
Phase III Trials
Phase IV Trials
Placebo
Placebo Effect
Pre-clinical Trials
Prevention Trials
Quality of Life Trials
Randomisation
Randomised Trial
Risk-benefit analysis
Screening Trials
Single-blind Trial
Trial endpoint
Treatment Trials
Adverse Event
Adverse Reaction