. 2023 Feb 16;15(2):e35077. doi: 10.7759/cureus.35077

Table 1. The phases, types, and nature of clinical trial studies.

This table has been created by the authors.

MTD: maximum tolerated dose; SAD: single ascending dose; MAD: multiple ascending doses; NDA: new drug application; FDA: food and drug administration

Clinical trial phase	Type of the study	Nature of study
Phase 0	Exploratory	Examines too low (1/100^th) concentrations (micro-dosing) of the drug for less time. Study the pharmacokinetics and determine the dose for phase I studies. Previously done in animals but now it is carried out in humans.
Phase I, Phase Ia, Phase Ib	Non-therapeutic trial	Around <50 healthy subjects are recruited. Establishes a safe dose range, and the MTD. Examines the pharmacokinetic and pharmacodynamic effects. Usually single-center studies. Phase Ia: SAD, and MTD. Duration of one week to several months depending on the trial and includes 6-8 groups of 3-6 participants. Phase Ib: MAD and the dose is gradually narrowed down. Three groups of 8 individuals each.
Phase II, Phase IIa, Phase IIb	Exploratory trial	Recruiting around 5-100 patients of either sex. Examines the effective dosage and the therapeutic effects on patients. It decides the therapeutic regimen and drug-drug interactions. Usually, multicentre studies. Phase IIa: Decides the drug dosage, includes 20-30 patients, and takes up to weeks/months. Phase IIb: Studies dose-response relationship, drug-drug interactions, and comparison with a placebo.
Phase III	Therapeutic confirmatory trial	More than 300 patients (up to 3000) of either sex are recruited in this study and are multicentric trials. Pre-marketing phase examines the efficacy and the safety of the drug. Comparison of the test drug with the placebo/standard drug. Adverse drug reactions/adverse events are noted. Initiate the process of NDA with appropriate regulatory agencies like the FDA.
Phase IV	Post-approval study	After approval/post-licensure and post-marketing studies/surveillance studies. Following up on the patients for an exceptionally long time for potential adverse reactions and drug-drug interactions.