A clinical study in which the participants are allocated randomly to receive an experimental clinical intervention (such as a new antibiotic) or a control intervention. RCTs are considered the most rigorous test of efficacy of a new treatment.

The control intervention may be a placebo or the usually used treatment for a condition (standard of care).

Ideally, neither patient nor experimenter should know which treatment is being given (a double-blind RCT). If it is not possible for the experimenter not to be aware of the treatment, a single-blind design is used.

In open-label trials, both patient and experimenter are aware which treatment is being provided. This creates a significant risk of bias. Various methods are used to reduce the risk of bias, such as assessment of outcomes by an individual who does not know which treatment was given.