Step 1 topics → General Pharmacology → Therapeutic Index (TI)

Therapeutic Index (TI)

USMLE Step 1 trap: Inverts the therapeutic index formula (ED50/LD50 instead of LD50/ED50). The therapeutic index is LD50 divided by ED50; a higher TI means a safer drug.

The therapeutic index (TI) is one of those concepts that looks simple on the surface but hides a few traps the USMLE Step 1 loves to exploit. The formula is TI = LD50/ED50 — the ratio of the lethal dose to the effective dose in a population. A high TI means you have a lot of breathing room between the dose that works and the dose that kills. A low (narrow) TI means that margin is razor-thin, which is why those drugs require close monitoring in clinical practice. The exam tests this at two levels: pure definition recall and clinical application (which drugs are narrow-TI and what that means for patient management).

The trickiest part isn't the formula itself — it's knowing what the formula actually tells you. Students frequently invert it (ED50/LD50), which flips the safety interpretation entirely. A ratio less than 1 would be nonsensical for a usable drug, so if your formula gives you that, you've got it backwards. The other common trap is conflating a wide TI with 'safe drug, no worries.' That's wrong. Wide TI means a large dose margin before lethality — it says nothing about side effects, organ toxicity, or drug interactions at therapeutic doses.

On USMLE Step 1, this concept shows up most often in pharmacology vignettes where a patient is on a drug like digoxin, warfarin, lithium, or phenytoin, and a clinical decision about monitoring or dosing adjustment is being tested. Recognizing the narrow-TI drug list and understanding *why* those drugs need serum level checks is the applied layer the exam expects you to have ready.

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Common misconceptions

Common mistake
Wrong: The therapeutic index is calculated as ED50 divided by LD50.
Right: The therapeutic index is LD50 divided by ED50; a higher TI means a safer drug.
The correct formula is TI = LD50/ED50, not ED50/LD50. Think of it as: how many times the effective dose can you go before you hit the lethal dose? A higher number means more safety margin. If you invert it, a 'safer' drug would paradoxically give a smaller number, which breaks the logic entirely — always keep LD50 in the numerator.
Common mistake
Gap: Fails to identify which drugs require serum level monitoring due to narrow therapeutic index
Narrow-TI drugs (e.g., warfarin, digoxin, lithium, phenytoin, aminoglycosides, cyclosporine) require routine serum level monitoring because small dose changes cause toxicity or therapeutic failure.
Narrow-TI drugs sit in a zone where the therapeutic dose and the toxic dose are very close together, so even minor fluctuations in drug levels — from renal function changes, drug interactions, or patient adherence — can tip the balance toward toxicity or sub-therapeutic failure. The classic list to memorize: Warfarin, Digoxin, Lithium, Phenytoin, Aminoglycosides, Cyclosporine (and theophylline). These require serum level monitoring precisely because the TI is so narrow that clinical judgment alone isn't sufficient.
Common mistake
Wrong: A wide therapeutic index means the drug has no serious side effects.
Right: A wide TI means there is a large margin between the effective and lethal dose, but the drug can still have significant side effects unrelated to the TI.
TI only measures the ratio between lethal and effective doses — it tells you nothing about adverse effects that occur at therapeutic concentrations. A drug can have a wide TI (hard to accidentally kill someone with it) and still cause serious organ damage, allergic reactions, or significant drug interactions at normal doses. For example, acetaminophen has a relatively wide TI in healthy patients but causes severe hepatotoxicity at overdose through a mechanism entirely separate from its TI. Don't use TI as a proxy for overall drug safety.
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What the exam tests

  1. Define the therapeutic index mathematically (TI = LD50/ED50) and explain what a higher versus lower TI means for drug safety in practice.
  2. Identify which specific drugs have a narrow therapeutic index — including warfarin, digoxin, lithium, phenytoin, aminoglycosides, and cyclosporine — and recognize that these require routine serum level monitoring because small dose changes can cause toxicity or treatment failure.

Can you avoid these mistakes?

A drug has an ED50 of 10 mg/kg and an LD50 of 200 mg/kg. What is its therapeutic index, and would you classify this drug as narrow or wide TI? What would the TI be if the values were reversed?
A patient on digoxin develops nausea, visual changes, and bradycardia after being started on a new medication. Why does digoxin's therapeutic index make this patient particularly vulnerable to drug interactions, and what lab value would you check?
Your attending says a drug has a 'wide therapeutic index,' and a classmate concludes it must be safe to prescribe without much monitoring. What is wrong with that reasoning, and what does TI actually tell you (and not tell you)?
List at least five narrow-TI drugs from memory. For each one, name one clinical scenario (e.g., a specific organ dysfunction or drug interaction) that could push the drug level into the toxic range.

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