Phase II clinical trials involve 50-500 subjects and aim to confirm effectiveness and further evaluate safety of an experimental drug. They can last 6 months to several years. Phase II trials may be divided into Phase IIa to assess dosing requirements and Phase IIb to assess efficacy. Phase II trials now commonly involve a control group receiving either standard treatment or placebo for comparison. Phase III trials involve several hundred to 3000 subjects and are pivotal for obtaining marketing approval by confirming safety and effectiveness of the drug based on randomized, controlled, and often blinded studies run across multiple sites. Phase IV trials monitor effectiveness and side effects of approved drugs during real-world use.