PlainMeds guide

Narrow Therapeutic Index Drugs

FDA bioequivalence and interaction-record context for medicines with narrow exposure ranges.

A narrow therapeutic index (NTI) drug has a small gap between measured concentrations associated with effect and harm, so FDA labels give interaction and bioequivalence context special weight.

Narrow therapeutic index drugs, such as warfarin, levothyroxine, and lithium, have a narrow exposure range. This guide explains FDA bioequivalence and label-record concepts. PlainMeds does not provide personal dosing, substitution, monitoring, or treatment directions.

Key Takeaway

Narrow therapeutic index (NTI) drugs have a small gap between an effective dose and a toxic one. FDA labels and related guidance can therefore include interaction, concentration, and bioequivalence information that differs from standard generic records.

What Is a Therapeutic Index?

Every drug has a therapeutic window, a range of blood concentrations that is effective without being toxic. The therapeutic index (TI) measures how wide that window is. A high TI means there's a large margin of safety between a therapeutic dose and a toxic one. A low TI means the window is narrow.

A narrow therapeutic index is a source-record concept used in FDA bioequivalence and label materials. It does not establish a dosing, monitoring, or substitution rule in this registry.

How the FDA Identifies NTI Drugs

The FDA does not maintain a single official NTI drug list, the classification is pharmaceutical industry terminology incorporated into FDA guidance documents. The FDA uses the concept when evaluating generic drug applications, particularly for drugs where it applies the Reference-Scaled Average Bioequivalence methodology, which uses tighter acceptance criteria.

Several FDA guidance documents and Orange Book listings identify drugs subject to additional bioequivalence scrutiny, including levothyroxine sodium, warfarin, cyclosporine, and tacrolimus.

The Standard Bioequivalence Requirement

Standard generic drugs must demonstrate that their maximum plasma concentration (Cmax) and total drug exposure (AUC) fall within 80–125% of the brand name drug's values, with 90% confidence. This allows considerable variation in individual patients, but for most drugs with wide therapeutic windows, this variation is clinically insignificant.

For some NTI evaluations, the FDA applies a tighter criterion, typically 90–111% - meaning generic applicants must demonstrate closer similarity to the reference drug. The source record does not establish a substitution decision for an individual.

NTI source-record categories

The registry can link product records to FDA label fields and Orange Book or bioequivalence guidance. It does not publish therapeutic targets, monitoring intervals, interaction-response rules, or a product selection table.

What the source records cover

The FDA label and related source records can identify product-specific interaction risks, laboratory concepts, contraindications, and bioequivalence language. Those fields describe the record for a medicine. They are not a monitoring schedule, a substitution rule, or a treatment plan for the person reading the page.

Looking Up Drug Interaction Data

Every prescription drug page on PlainMeds includes interaction data sourced from FDA drug labels. The drug database lists the interactions recorded in official FDA labeling. The interaction checker contains the source-record pairing data available in this registry.

The alternatives database groups FDA-sourced therapeutic alternatives by drug class; it does not recommend a substitute.

Frequently Asked Questions

What makes a drug "narrow therapeutic index"?
A narrow therapeutic index (NTI) drug is one where the toxic dose is close to the therapeutic dose, the window between "not enough" and "too much" is very small. The FDA defines NTI drugs as those where a small change in dose or blood level can lead to serious therapeutic failure or adverse reactions. Examples include warfarin, digoxin, lithium, cyclosporine, and phenytoin.
Are generic versions of NTI drugs as safe as brand-name versions?
The FDA applies tighter bioequivalence criteria to some NTI products. Standard generic applications commonly use an 80–125% interval for exposure measures, while some NTI evaluations use a narrower 90–111% interval. Product-level labeling and FDA guidance describe the applicable record; PlainMeds does not direct substitutions for an individual.
What are the signs of NTI drug toxicity?
FDA labels contain product-specific adverse-reaction and warning fields. This registry does not turn those fields into symptom interpretation or a response instruction for an individual.
How do drug interactions affect NTI drugs differently?
Because NTI drugs operate within a narrow range, factors that change measured drug levels can have a larger effect than they do for other medicines. FDA labels and source records identify interactions, but PlainMeds does not determine monitoring or treatment for an individual.
How often should NTI drug levels be monitored?
Monitoring schedules vary by product, indication, clinical history, and the source protocol. This registry records that monitoring may appear in FDA labels and related guidance, but it does not publish a schedule for an individual.
What foods affect NTI drugs?
FDA labels can include food or interaction language for a specific product. PlainMeds preserves the source-record category and does not provide dietary, timing, or medication directions.

Research your medications

Browse FDA drug labels and interaction records in this registry.

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Information sourced from FDA drug labels, FDA Orange Book, and FDA bioequivalence guidance. This registry does not provide patient-specific medication, treatment, substitution, monitoring, or dietary directions. Spot an error in this guide? Report a correction.

Narrow-therapeutic-index medications covered

Warfarin Levothyroxine Lithium Digoxin Phenytoin Theophylline

Open any medication for its full FDA profile: labeled uses, side effects, interactions, recalls, and pricing.

Sources

Source: U.S. Food and Drug Administration: drug labeling and bioequivalence guidance for narrow therapeutic index drugs.

More plain-English guides

Understanding Drug Interactions → How Drug Interactions Work → Pain Relievers Compared → Generic vs Brand-Name Drugs → FDA Drug Recalls Explained → How to Read an FDA Drug Label →