Narrow therapeutic index
A narrow therapeutic index (NTI) drug is one where small differences in dose or blood concentration may lead to serious therapeutic failures or to adverse reactions that are life-threatening or cause persistent or significant disability, as the FDA defines it.
What it means in practice
The index compares the amount of a drug that works with the amount that harms. US bioequivalence regulations give a working test for a narrow therapeutic ratio: less than a 2-fold difference between the median lethal dose and the median effective dose, or between the minimum toxic and minimum effective concentrations in the blood, where safe and effective use requires careful dosage titration and patient monitoring [3].
The FDA formed an NTI Drug Working Group in 2015 that evaluates candidate drugs against five general characteristics, using theophylline products as a case example [2]. As of January 5, 2024, 33 drug products with 14 distinct active ingredients were specified as NTI drugs in FDA product-specific guidances for generic drugs, according to a review by FDA staff [2]. The authors suggest that working with other agencies to harmonize NTI terms and definitions may help clarity [2].
Why it matters when reading nootropics research
The concern for supplements is a change in the blood level of a medicine whose safe range is small: a shift that would not matter for most medicines can matter for these. NootropicsWork's safety-flag criteria therefore count an interaction reference that names a narrow-index medicine class, such as anticoagulants and antiplatelets, antiepileptics, immunosuppressants or digoxin.
In a 2012 systematic review of documented supplement interactions, warfarin and digoxin were among the medicines with the most reported interactions with herbs and dietary supplements [1]. Compound pages name such medicines only when a cited source names them, and refer readers who take them to a doctor or pharmacist.
References
- Tsai HH, Lin HW, Simon Pickard A, et al. Evaluation of documented drug interactions and contraindications associated with herbs and dietary supplements: a systematic literature review. Int J Clin Pract. 2012.
- Donnelly M, Fang L, Madabushi R, et al. Narrow Therapeutic Index Drugs: FDA Experience, Views, and Operations. Clin Pharmacol Ther. 2025.
- U.S. Office of the Federal Register (eCFR). 21 CFR 320.33 Criteria and evidence to assess actual or potential bioequivalence problems. eCFR up to date as of 2026-11-09.