In which phase of clinical studies is an investigational drug typically tested on 5 to 20 volunteer patients?

The phase of clinical studies in which an investigational drug is typically tested on 5 to 20 volunteer patients is called Phase I.

1. **Preclinical Trials**: This stage involves laboratory research and tests on animal subjects to assess the drug’s safety and biological activity.

2. **Phase I Trials**: This is the first phase of testing in human subjects. Phase I trials aim to determine the drug's safety profile, mode of action, side effects at increasing doses, and, if possible, early signs of effectiveness. This phase involves a small group of healthy volunteers (usually between 20 and 80, but sometimes as few as 5 to 20) and is primarily concerned with assessing safety and tolerability rather than efficacy.

3. **Phase II Trials**: If the drug is found to be safe in Phase I, it moves to Phase II where it is given to a larger group of patients (usually 100 to 300). These subjects often have the condition that the drug is intended to treat. Phase II aims to determine the drug's efficacy, optimal dosages, and further assess safety.

4. **Phase III Trials**: In this phase, the drug is given to much larger groups of people (usually 1,000 to 3,000) to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect all the information necessary to ensure safe usage.

5. **Phase IV Trials**: These are post-marketing studies conducted after a drug has been approved for consumer sale. Phase IV studies provide additional information including the drug's risks, benefits, and optimal use.