Therapeutic Index
Also written as TI.
Pharmacodynamics is what the drug does to the body (receptors, dose-response, therapeutic index).
What our notes say
- Pharmacodynamics is what the drug does to the body (receptors, dose-response, therapeutic index). (Introduction to Psychopharmacology)
- It has a narrow therapeutic index, so trough levels are monitored. (Clinical Pharmacology I (MBPL 4400) main exam: 25 January 2023)
- Its limitation is that it does not improve survival (it only reduces hospital admissions) and has a narrow therapeutic index (toxicity, worse with hypokalaemia). (Clinical Pharmacology I (MBPL 4400) main exam: 25 January 2023)
- Digoxin has a narrow therapeutic index and is excreted by the kidneys. (Pharmacology Question Bank: 15 Clinical MCQs)
- Lithium has a narrow therapeutic index and needs blood levels. (Introduction to Psychopharmacology)
- Toxicity (narrow therapeutic index): coarse tremor, vomiting, diarrhoea, ataxia, dysarthria, confusion, seizures. (Bipolar and Related Disorders)
Study material on Therapeutic Index
- Introduction to Psychopharmacology
- Weekly Pharmacology Exam - April 21, 2026 (Section A: MCQs)
- Weekly Pharmacology Exam - April 17, 2026 (Section A: MCQs)
- COMPREHENSIVE BASIC PHARMACOLOGY - 120 CHALLENGING MCQs
Practice questions on Therapeutic Index
Therapeutic index (TI) indicates:
- Margin of safety
- Rate of clearance
- Genetic polymorphism
- Binding affinity
Answer: A. TI = TD50/ED50 or LD50/ED50. A higher TI indicates a wider margin between therapeutic and toxic doses, meaning greater safety. ---
A drug has an ED50 of 10mg and a TD50 of 100mg. What is its therapeutic index?
- 0.1
- 90
- 1
- 10
Answer: D. Therapeutic Index = TD50/ED50 = 100/10 = 10. A higher therapeutic index indicates a wider margin of safety between therapeutic and toxic doses. ---
Which drug has the narrowest therapeutic index requiring routine TDM?
- Omeprazole
- Digoxin
- Metformin
- Amoxicillin
Answer: B. Digoxin has a very narrow therapeutic window (0.5–2 ng/mL); small concentration changes cause toxicity or therapeutic failure. ---
: The LD50 of a new compound is determined to be 200 mg/kg, while its ED50 is 20 mg/kg. What is the Therapeutic Index (TI) of this drug?
- 0.1
- 10C) 180
- 220
- 10
Answer: B. The Therapeutic Index is calculated as LD50/ED50. In this case: 200/20 = 10. A TI of 10 indicates the drug has a reasonable margin of safety, as the lethal dose is 10 times higher than the effective dose. Drugs with higher TI values are generally considered safer. ---
The LD50 of a new compound is determined to be 200 mg/kg, while its ED50 is 20 mg/kg. What is the Therapeutic Index (TI) of this drug?
- 0.1
- 180
- 10
- 220
Answer: C. Therapeutic Index (TI) = LD50 / ED50. In this case, 200 / 20 = 10.
A drug with TD50 of 150 mg and ED50 of 50 mg would have what Therapeutic Index?
- 100
- 7500
- 3
- 0.33
Answer: C. TI = TD50 (Toxic Dose) / ED50 (Effective Dose). 150 / 50 = 3.