Drug Interaction Report

Cholestyramine and Levothyroxine: Interaction Details

A pharmacist-reviewed analysis of the Cholestyramine and Levothyroxine interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Cholestyramine

Prevalite Questran
+

Levothyroxine

Ermeza Euthyrox Levo-T Levo-T® Levothroid® Levoxyl Levoxyl® Synthroid
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Moderate
Can be significant — usually manageable with monitoring.
How we grade severity & evidence

Severity levels

  • Contraindicated: These should generally not be used together.
  • Major: Potentially serious — often needs a change or close monitoring.
  • Moderate: Can be significant — usually manageable with monitoring.
  • Minor: Usually limited clinical impact.

Evidence grades

  • Established: Well documented — supported by controlled studies or strong clinical data.
  • Probable: Good supporting evidence, though not definitively proven.
  • Suspected: Some evidence suggests this interaction, but it is not well established.
  • Possible: Limited or conflicting evidence; the interaction may occur.
  • Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.

Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.

Of 40 documented Cholestyramine interactions, 23 are rated moderate — including this one.
Onset
rapid
Evidence
probable
Severity
Moderate

What happens

Reduced efficacy of levothyroxine and an increased risk of hypothyroidism

Interaction Deep Dive

Concomitant use of cholestyramine and levothyroxine may decrease levothyroxine efficacy by binding to and delaying or preventing levothyroxine absorption, potentially resulting in hypothyroidism32. Avoid concomitant use if possible. If coadministration is required, maximally separate administration 1 and administer levothyroxine at least 4 hours prior to cholestyramine. Monitor TSH levels 32.

Why it happens (mechanism)

Binding with levothyroxine; reduced levothyroxine absorption

Literature reports

1 report — tap to read

a) Significant elevations in TSH developed when cholestyramine was administered concomitantly with levothyroxine in a 51-year-old patient with hypothyroidism. TSH levels normalized following the discontinuation of cholestyramine. A dosing interval of at least 6 hours should separate the administration of bile acid sequestrants and levothyroxine 4.

Common questions

Can I take Cholestyramine and Levothyroxine together?

Reduced efficacy of levothyroxine and an increased risk of hypothyroidism Always confirm with your pharmacist or prescriber before making any change.

How serious is the Cholestyramine and Levothyroxine interaction?

It is rated moderate. Can be significant — usually manageable with monitoring.

How quickly could this interaction happen?

The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.

How strong is the evidence for this interaction?

The evidence is graded "probable". Good supporting evidence, though not definitively proven.

Questions for your pharmacist

  • Does my dose of Cholestyramine or Levothyroxine need adjusting while I take them together?
  • What symptoms should prompt me to call you or my prescriber right away?
  • Does the timing of my doses matter for this combination?
  • Is there anything you'd monitor while I'm on both?

References (4)

  1. Centanni M, Duntas L, Feldt-Rasmussen U, et al: ETA guidelines for the use of levothyroxine sodium preparations in monotherapy to optimize the treatment of hypothyroidism. Eur Thyroid J 2025; 14(4):e250123. PubMed
  2. Product Information: TIROSINT(R)-SOL oral solution, levothyroxine sodium oral solution. IBSA Pharma Inc. (per FDA), Parsippany, NJ, 2023. DailyMed
  3. Product Information: SYNTHROID(R) oral tablets, levothyroxine sodium oral tablets. AbbVie Inc (per FDA), North Chicago, IL, 2024. DailyMed
  4. Harmon SM & Seifert CF: Levothyroxine-cholestyramine interaction reemphasized. Ann Intern Med 1991; 115:658-659. PubMed
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Cholestyramine

Keep reading about Cholestyramine

All Cholestyramine interactions (40)
Also serious with: Deferasirox Dexamethasone Injection Dexamethasone Ophthalmic Digoxin
Levothyroxine

Keep reading about Levothyroxine

All Levothyroxine interactions (214) Levothyroxine drug information
Also serious with: Furosemide Polystyrene Sulfonate Insulin Glargine, Recombinant Ferrous Sulfate
Beyond drug–drug

These medications also interact with supplements

Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:

Cholestyramine + Activated Charcoal Coffee Charcoal European Mandrake Glucomannan Guar Gum Gum Arabic Niacin Slippery Elm + 11 more
Levothyroxine + Activated Charcoal Tiratricol Acetyl-l-carnitine Alpha-lipoic Acid Ascophyllum Nodosum Ashwagandha Bugleweed Calcium + 42 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.