A clinical trial is the testing of a pharmaceutical or other medical procedure under controlled conditions to ensure that the treatment is both safe and effective. It usually follows preliminary human trials where the treatment is given to affected patients to see if they improve after treatment.

When a drug is approved to be tested on humans, it is first tested on those who suffer from the condition the treatment is supposed to assist. However, even successful preliminary trials cannot prove that a drug is safe and effective. The perceived benefit of the treatment may be illusory (perhaps by patients recovering spontaneously) or the treatment may have long lasting side effects that do not exist with other drugs used to treat the condition.

A clinical trial differs from a preliminary trial as follows:

  • It is controlled. Half the participants receive a placebo instead of the active treatment. This determine whether the treatment actually is having an effect.
  • It is double blind. Neither the physicians doing the assessment of the treatment's effects nor the patient know whether they are in the treatment group or the control group.
  • It is long term. A clinical trial usually lasts about five years. This helps to determine if they drug has any long term side effects.

Many common treatments have never had a clinical trial, such as the heart bypass. Many other drugs (such as aspirin) have been in use for so long their effectiveness and relative safety have been established without clinical trials. Many drugs have done well in clinical trials, only to have their side effects missed during this period of time in any event (Vioxx, thalidomide). Some drugs have shown such an incredible effect so early on that it was judged unethical to withhold them from the control group (AZT).

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Clinical trial at Wikipedia