Clinical trials are conducted in phases, with each phase designed to answer different questions about a treatment’s safety, effectiveness, and long-term value. These phases help ensure that new drugs or medical devices are safe and work as intended before they reach the general public.

Phase 0/Early Phase 1

• Involves a very small number of participants (usually fewer than 15).
• Tests how a new drug behaves in the body, how it is absorbed, processed, and eliminated.
• Uses very small doses and does not aim to treat the condition.
• Helps decide whether further testing is worthwhile.

Phase 1

• Typically includes 20 to 100 healthy volunteers or patients.
• Focuses on safety: What is the safe dosage range? What side effects might occur?
• Determines how the body reacts to the drug or treatment.
• May involve dose-escalation to identify the highest safe dose.

Phase 2

• Involves several hundred participants with the condition being studied.
• Tests whether the treatment works as intended (effectiveness).
• Continues monitoring safety and side effects.
• Often compares different doses or treatment schedules.
• Usually randomized and may involve a placebo group.

Phase 3

• Includes hundreds to thousands of participants.
• Confirms how well the treatment works in a broader group.
• Compares the new treatment with standard treatments or placebo.
• Collects data needed for regulatory approval by agencies like the FDA, EMA, or CDSCO.
• Tracks both benefits and risks over a longer period.

Phase 4

• Conducted after the treatment is approved and made available to the public.
• Monitors long-term effectiveness, side effects, and safety in real-world use.
• May identify rare or delayed effects that did not appear in earlier phases.
• Also called post-marketing surveillance.

Key Note: The number of participants in each phase can vary depending on the condition being studied and the treatment being tested.