Supplement · Grade A

L-Carnitine and medications.

Every documented pair, every citation. Below: 3 documented pairs grouped by mechanism.

L-Carnitine is in the Distil supplement database, evidence Grade A. The page below lists every medication we have explicitly assessed it against.

L-carnitine shuttles fatty acids into mitochondria to be burned for energy, and the form you take decides what it does. The acetyl form, ALCAR, crosses the blood-brain barrier and is studied for cognitive function and neuroprotection; the tartrate form targets exercise performance, recovery and male fertility through sperm motility. The evidence is Grade A for male fertility and Grade B for cognitive, energy and cardiovascular effects. The non-negotiable point is form: never take unspecified L-carnitine, since ALCAR and the tartrate are not interchangeable. Doses are roughly 500 to 2,000mg ALCAR for cognition and 1 to 3g tartrate for athletic or fertility goals. Vegans get no carnitine from diet, so it is more relevant for them. It may alter how thyroid medication behaves and warrants monitoring alongside anticoagulants, so flag both to your prescriber. A fishy odour can appear at high doses. Pick the form that matches your actual goal.

Below are the 3 documented pairs we have explicitly assessed for L-Carnitine: 3 amber. The pairs cluster around 2 mechanisms: Additive anticoagulation and Reduced thyroid-hormone effect. Every call is cited to either a clinical reference (PMID) or the British National Formulary. Anything not listed here is either still to be assessed or beyond our database scope. The checker beneath surfaces assessments by medication, and the missing-item form at the bottom of the page routes any uncatalogued medication into our next curation pass.

Documented interactions

Additive anticoagulation

Amber Acenocoumarol

L-carnitine has been reported in a few cases to increase the effect of vitamin-K-based blood thinners like acenocoumarol, raising the INR. The evidence is limited. If you take acenocoumarol and use L-carnitine, mention it to your anticoagulant clinic and have your INR checked.

PMID 15340883 · BNF: Acenocoumarol

Reviewer-flagged: awaiting clinical-reviewer sign-off.

Amber Warfarin sodium

There is a single report of L-carnitine strengthening the effect of a warfarin-type blood thinner, raising the risk of bleeding. The report involved a closely related drug rather than warfarin itself, so the evidence is thin, but the drugs work the same way. If you take warfarin, mention L-carnitine to your anticoagulation clinic and ask whether an INR check is worth doing after you start.

PMID 15340883 · BNF: Warfarin

Reviewer-flagged: awaiting clinical-reviewer sign-off.

Reduced thyroid-hormone effect

Amber Levothyroxine

L-carnitine can blunt the effect of your thyroid hormone, because it slows the hormone getting into your cells. If you take levothyroxine, supplemental L-carnitine may work against it and let some underactive-thyroid symptoms creep back. Tell the clinician who manages your thyroid before starting it, and ask for a TSH check a few weeks later so your dose can be reviewed if needed.

PMID 11502782 · PMID 11201848 · BNF: Levothyroxine

Reviewer-flagged: awaiting clinical-reviewer sign-off.

What this list does not say. Pairs not flagged here are not implicitly safe. They are either not yet in our database, or fall outside our inclusion scope. Use the checker below to surface any medication, and submit a missing item if you take something we have not catalogued.

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For adults over 18. This tool gives evidence-graded information, not medical advice. Always discuss changes with your GP, especially if you take any medication, are pregnant, breastfeeding, or have a serious health condition.

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