Narrow Therapeutic Index Calculator
The formula
What the ti / nti tells you
The population margin between half-effective and half-toxic doses — and the FDA’s narrow-index definition that tightens bioequivalence.
Calculation: TI = TD/ED; NTI at ≤2; individual margin = (1±CV) reach of exposure. Enter your inputs and every substitution is shown below the result.
Reading the result
NTI is a formulation claim, not a moral panic: the drug is only narrow under the right monitoring.
Rules of thumb: NTI: levothyroxine, warfarin, phenytoin, tacrolimus, cyclosporine
Limitations and best practice
TI is derived from population ED/TD50 curves; your patient sits somewhere on the sigmoid and a level tells you where.
This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Frequently asked questions
What changes for NTI drugs?
Is NTI the same as toxic?
Source & methodology
FDA Orange Book NTI list; EMA 2010.
How Narrow Therapeutic Index works — and what your result really means
This page was written for one job: turning your own inputs into a defensible figure, with the working shown. Searchers land on a Narrow Therapeutic Index page for three reasons — to plan, to compare options, and to check that somebody else’s arithmetic is honest. This tool serves all three, and everything after the calculator explains the model instead of hiding it.
Health figures in Health & Medical describe populations, not individuals — Narrow Therapeutic Index places you on a distribution built from bodies like yours, and the 3.14 answer is a starting point for a conversation, not a diagnosis. That framing is the difference between using a health metric and being misled by one.
People rarely disagree about arithmetic; they disagree about assumptions that were never written down. Printing the substitution and the sensitivity ranking removes that category of disagreement for Narrow Therapeutic Index.
What makes a calculation page trustworthy is that its copy is derived from its own arithmetic: here therapeutic index = 3.14, recomputed every time a field changes, and the sensitivity section ranks the inputs that move the result — ED50 first, at ≈11.1 % of the headline for a ten-percent error either way in that input. Work down the page and by the end you could re-derive the result in a spreadsheet from memory.
Every input in Narrow Therapeutic Index, and what it does to the answer
The engine implements a single stated model: TI = TD/ED; NTI at ≤2; individual margin = (1±CV) reach of exposure. There is no hidden coefficient and no login-gated precision — the formula printed is the formula executed, which is also what makes the result citable.
TD50 (mg/L) (example value 22) is a numeric input whose unit is fixed by the label; a wrong unit is a wrong answer wearing a plausible disguise. Double-check that the unit on your source matches the unit this field expects before typing.
ED50 (mg/L) (example value 7) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Double-check that the unit on your source matches the unit this field expects before typing.
PK variability CV (%) (example value 26) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Rates are the most mis-transcribed input on any page like this; copy it from the document rather than retyping from memory.
Generic substitution allowed? (0/1) (example value 1) is a measured number in the unit named by the label; the formula does not convert for you. Replace the example with the figure from your own records — the tool recomputes instantly and the printed steps follow your numbers.
Narrow Therapeutic Index on the example inputs: the complete substitution
Abstract formulas are hard to trust, so here is the complete substitution using the default inputs, written the way an examiner would want it. Starting figures: TD50 (mg/L), ED50 (mg/L), PK variability CV (%), Generic substitution allowed? (0/1). The computed therapeutic index comes out at 3.14, which the interpretation band labels “TI 3.1 — conventional index; standard BE acceptance applies”.
- TI = TD50/ED50 = 22.0/7.0 = 3.14
- Variability: ED50×(1+2×0.26) = 10.6
- Overlap with TD50 — tail stays under TD50
Exposure +2CV point — 10.6 mg/L. This is the number experienced users quote when the headline alone would mislead.
Margin to TD — 11.4 mg/L. This is the number experienced users quote when the headline alone would mislead.
NTI class — standard. If your inputs change, this is usually the first companion figure to move.
BE window needed — 80.00–125.00%. This is the number experienced users quote when the headline alone would mislead.
Re-running with your own figures is instant; re-running by hand once is the best way to know the tool is telling you the truth.
How far Narrow Therapeutic Index can reasonably move
The honest version of any estimate is a corridor, and this one is cheap to produce: Narrow Therapeutic Index was recomputed at 5 points along the dominant input, with every other field left untouched.
| ED50 setting | Therapeutic index |
|---|---|
| 80 % of the example value | 3.93 |
| 90 % of the example value | 3.49 |
| example inputs | 3.14 |
| 110 % of the example value | 2.86 |
| 120 % of the example value | 2.62 |
Across 5 settings of ED50, the result spans 2.62 to 3.93. A decision that only works at one row of this table is a decision waiting for an input to move. Reproduce it in the form above by editing ED50 alone; the substitution steps will follow your figures exactly.
What actually moves your Narrow Therapeutic Index
This list was computed for this specific engine just now: each numeric input was nudged ±10 % while every other field held its example value, and the therapeutic index was recalculated from scratch. Read it as a map of where Narrow Therapeutic Index is fragile — drawn for the example inputs, which is the map you want when deciding what to measure carefully.
- Nudge ED50 by ±10 % and Narrow Therapeutic Index lands between 2.86 and 3.49 (sensitivity ≈11.1 %)
- TD50 +10 % → Narrow Therapeutic Index = 3.46 · −10 % → 2.83 (the therapeutic index shifts ≈10.0 %)
ED50 is the dominant lever here: ten percent either way moves the result by about 11.1 %, from 2.86 to 3.49. That is where a careless figure does the most damage, so check it against a source, and treat TD50 as the second priority (≈10.0 % per ten percent). The remaining fields can sit at safe defaults without materially changing the answer — knowing which is which is what separates a careful calculation from an anxious one.
What a Narrow Therapeutic Index of 3.14 means in practice
The value above only becomes useful the moment you know what it is being measured against. The default case lands in the “TI 3.1 — conventional index; standard BE acceptance applies” band, and bands are deliberately wide because real situations overlap. The sensitivity list shows how far a modest input error moves you along that ladder: for ED50, ±10 % covers 2.86 to 3.49. The practical reading rules: read the band as a starting hypothesis, then test it: change the two dominant inputs by ten percent and see whether the verdict survives. Verdicts that survive stress are the ones worth acting on.
Where Narrow Therapeutic Index sits next to similar measures: this tool answers “how much is the Narrow Therapeutic Index”, while the related calculators answer “what do I do about it”. Reading the result alone tends to over-weight a single snapshot; pairing it with the companion tools above turns the number into a decision.
Why Narrow Therapeutic Index results go wrong — and how to prevent it
TI is derived from population ED/TD50 curves; your patient sits somewhere on the sigmoid and a level tells you where. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Confusing a large number with an important one. Narrow Therapeutic Index can be dominated by one input whose absolute value looks unimpressive, which is why this page ranks influence separately from magnitude.
Attention is also misallocated between fields. Having measured this engine, we can say where it belongs: ED50 first, everything else second.
The subtlest error is answering the wrong question precisely. Confirm that therapeutic index is actually the quantity your decision needs — the arithmetic is flawless either way, which is what makes the mistake invisible.
Before you trust your Narrow Therapeutic Index: a short checklist
- Recompute once with the dominant input deliberately set low, and see whether the decision changes.
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- Re-verify ED50 against a source document — ten percent of error there moves the headline by ≈11.1 %.
- Compare the result with one independent method or published benchmark.
- Confirm the example values are plausible for your own situation before replacing them.
- Confirm the answer against one independent method or published benchmark for Narrow Therapeutic Index.
A routine for decision-grade Narrow Therapeutic Index
The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.
Step 1 — collect, then type. Everything this tool asks for is listed above (td50 (mg/l), ed50 (mg/l), pk variability cv (%), generic substitution allowed? (0/1)). Pull each figure from something you can point at — paperwork, a dashboard, a label — before touching a field. Guesswork at the input stage cannot be repaired at the interpretation stage. The test of a good estimate is not that it feels right but that it changes what you do.
Step 2 — enter the conservative case first. Resist the flattering scenario. Type the inputs you would defend in front of a sceptical reader, read the Therapeutic index (3.14 for the example values), and note the verdict band. The point of transparency is that a reader can disagree with a specific line rather than with you.
Step 3 — isolate your variables. Change one field, observe, revert. The live sensitivity list does the ranking for the default inputs; verify it against your own, because dominance shifts when the starting point shifts. A figure nobody can reproduce is a figure nobody will act on twice.
Step 4 — run the scenario trio. Expected, pessimistic, optimistic. The gap between pessimistic and expected is the number to plan against; the optimistic result is useful only as a ceiling. Write the figure down before moving on; memory flatters whichever number you saw last.
Step 5 — schedule the next run. In Health & Medical the inputs drift quietly. Put a reminder a quarter out, re-enter the same fields, and read the change rather than the level. A result worth acting on survives having its two shakiest inputs halved.
From calculation to decision: where Narrow Therapeutic Index earns its keep
Numbers earn their keep in conversations, and Narrow Therapeutic Index is no exception. In Health & Medical discussions around Medications & Dosing, this figure decides whether an option stays on the table: it converts an opinionated debate into an arithmetic one, and arithmetic is the only referee both sides accept. Write the figure down before moving on; memory flatters whichever number you saw last.
Professionals in Medications & Dosing use this style of calculation as a screening filter before spending real money or effort: a quick Narrow Therapeutic Index check eliminates weak options in minutes, and the survivors earn the deeper, slower analysis. Comparisons are cheap: run the identical calculation for the alternative and look at the gap.
Treat Narrow Therapeutic Index like a smoke alarm rather than a painting — its value is in being tested periodically, not in being admired once. Re-running a number costs seconds; retracting a decision costs considerably more.
Narrow Therapeutic Index — frequently asked questions
What affects Narrow Therapeutic Index the most?
Measured on this engine, ED50 is the dominant lever: moving it ±10 % shifts the therapeutic index across 2.86 to 3.49 — about 11.1 % of the headline, with TD50 next at ≈10.0 %. The ranked list in the sensitivity section is computed live, not asserted from a template.
Which matters more: td50 or ed50?
ED50, by a factor of about 1.1. Concretely: ten percent on ED50 is worth ≈11.1 % of Narrow Therapeutic Index, the same nudge on TD50 only ≈10.0 %. That ranking is re-computed for each page from that page’s own defaults.
What is a typical Narrow Therapeutic Index?
On the worked example above Narrow Therapeutic Index comes out at 3.14. Treat that as one point on a distribution rather than a norm: the bands, the companion outputs and the sensitivity ranking tell you how wide the plausible range really is.
How do I calculate Narrow Therapeutic Index by hand?
Three steps: write the formula (TI = TD/ED; NTI at ≤2; individual margin = (1±CV) reach of exposure), insert your values, and simplify. The worked example section shows exactly that chain for the default inputs; replicating it once on paper teaches you more about Narrow Therapeutic Index than any number of calculator runs.
What happens to Narrow Therapeutic Index if ed50 changes by 10 %?
On these inputs a ±10 % change in ed50 takes Narrow Therapeutic Index to 2.86 against the 3.14 example — around 11.1 % of the headline. The same test on every other field is in the sensitivity section.
Is the Narrow Therapeutic Index result of 3.14 good or bad?
Good and bad are comparisons, so the honest test is three questions: where does your own Narrow Therapeutic Index sit against the 3.14 reference above, how far would a plausible input error move it (≈11.1 % per ten percent on ED50), and what does the alternative option compute to?
How accurate is Narrow Therapeutic Index for me personally?
The formula encodes averages; body-composition and measurement error typically shifts results by a few percent in either direction. Use ranges and trends, and treat any single 3.14 as approximate.
How do I calculate Narrow Therapeutic Index manually?
Write down the inputs, then follow the printed steps in order — TI = TD/ED; NTI at ≤2; individual margin = (1±CV) reach of exposure. A spreadsheet replicates it with two or three cells. The manual route is worth doing once to build intuition; after that the calculator saves time and eliminates transcription slips.
What changes for NTI drugs?
Regulatory BE limits narrow, manufacturers should not switch casually, and monitoring tightens after any change.
Is NTI the same as toxic?
No — it measures the gap between curves, not the absolute hazard.
Does this Narrow Therapeutic Index tool store my numbers?
No. The entire calculation runs in your browser session; nothing entered above is transmitted or saved, which is the correct behaviour for sensitive Health & Medical figures and the reason you can re-run this page as often as you like.
The vocabulary of Narrow Therapeutic Index
- Exposure +2CV point
- 10.6 mg/L in the default Narrow Therapeutic Index calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Margin to TD
- 11.4 mg/L on the example inputs — a companion figure printed beside the headline of Narrow Therapeutic Index so that no result is read in isolation.
- NTI class
- standard on the example inputs — a companion figure printed beside the headline of Narrow Therapeutic Index so that no result is read in isolation.
- Therapeutic index
- The primary output of this tool: 3.14 for the default inputs, computed exactly from TI = TD/ED; NTI at ≤2; individual margin = (1±CV) reach of exposure.
- Worked example
- Every substitution in order, from the example inputs to 3.14 — the audit trail for the headline figure.
- Verdict band
- The interpretation attached to a result (“TI 3.1 — conventional index; standard BE acceptance applies” on these inputs), which turns a bare number into a position on a scale.
- Sensitivity (±10 %)
- How far the Narrow Therapeutic Index moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Methodology, sources and limits
There is no black box on this page — the engine evaluates TI = TD/ED; NTI at ≤2; individual margin = (1±CV) reach of exposure. Source of record: FDA Orange Book NTI list; EMA 2010. Any other implementation of this model should return the same figure for the same inputs. If one does not, the difference is in its assumptions — which this page prints and that one probably does not.
Nothing here replaces a qualified professional, and nothing here is approximate either — the arithmetic is exact, and the honesty is in the inputs you supply. What that buys you is reproducibility — two people, two browsers, the same arithmetic, and a printed trail from inputs to result that either of them can audit.
What to do with your Narrow Therapeutic Index next
Re-run the calculation with your own figures, stress the two most sensitive inputs by ±10 %, and only then act on the result.
Whoever you are (people who need one trustworthy number before their next decision), the next step is the same: replace the example inputs with your own, note the date, and re-run whenever a figure moves. In Health & Medical, a dated figure is a trend, and an undated one is a rumour.