The second step in testing an experimental drug (or other treatment) in humans. Typically, Phase II trials are done only if Phase I trials have shown that the drug is safe, but sometimes Phase I and Phase II trials are combined. Phase II trials are designed to evaluate the drug's effectiveness in people with the disease or condition being studied and to determine the common short-term adverse effects and risks associated with the drug. Phase II trials involve more participants (often several hundred) and typically last longer than Phase I trials.