Many older patients have multiple diseases that require high-risk drug use, which may cause serious adverse effects. Criteria for the use of these drugs in older people are vital to the prevention of adverse events. This study aimed to develop explicit criteria for determining high-risk medication use in Thai older patients. It was conducted using a Delphi technique with the three-round survey of 16 geriatric medicine (GM) experts. A structured questionnaire with 56, 93, and 95 statements were devised for Rounds 1, 2, and 3, respectively. In each statement, the panelists were requested to confirm a high-risk drug in Round 1, and to rate on a 5-point Likert scale and classify the high-risk medication use in Rounds 2 and 3. The results showed that 77 practice statements (81.1%) that embraced the use of high-risk medications with potential adverse reactions, drug-disease interactions, and drug-drug interactions were agreed by the expert panel. A total of 23 statements (29.9%) were categorized as Groups 1-3 and the rest remained unclassified. Most high-risk medications were utilized in the central nervous system, musculoskeletal system, and cardiovascular system. Further studies are warranted to evaluate the criteria in terms of prescribing and monitoring medication use in older patients.