Risk is the probability of harm or injury occurring as a result of using a treatment in clinical practice or as part of a research study. The harm or injury may be physical, but can also be psychological, social, or economic. Risks may include experiencing side effects of the treatment, or taking a medication that is not as effective as the standard treatment (during a trial). In a trial, a new treatment may have side effects or risks that researchers do not expect. This is more likely in the early stages of clinical trials.

No clinical trial is risk free. Participants should be aware of both the benefits and the risks before they make a decision about whether or not to take part (see informed consent).

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