Phase II is where a drug gets its first real test of efficacy. After Phase I establishes safety and a dose range, Phase II asks whether the drug actually works in patients with the target condition. These trials are larger than Phase I, usually 100 to 300 participants, and they often include a control group, either placebo or standard treatment. The design can be single-arm, where everyone gets the drug, or randomized, where participants are assigned to different groups.

The primary endpoint varies by disease. In oncology, it might be tumor response rate or progression-free survival. In cardiology, it might be a change in blood pressure or cholesterol. In psychiatry, it might be a change on a symptom rating scale. Phase II also continues to monitor safety, because rarer side effects may emerge when more patients are exposed. If the drug shows promise and the safety profile is acceptable, it moves to Phase III. If not, development stops. Roughly 30 percent of drugs that enter Phase II advance to Phase III.

What Phase II answers:

• Efficacy. Does the drug produce the intended effect?
• Dose-response. What dose works best?
• Safety. What side effects occur in patients?
• Endpoints. What outcome measures are most useful?

Phase II trials are often divided into IIa, which focuses on dosing and activity, and IIb, which focuses on efficacy at the selected dose.