L-Carnitine and medications.
L-Carnitine is in the Distil supplement database, evidence Grade B. 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 is the one studied for male fertility through sperm motility. The evidence is Grade B for cognitive effects and Grade C for male fertility. We lowered the fertility grade in September 2026, and the reason matters more than the letter. The trial our earlier grade rested on was one crossover study in 86 men measuring sperm concentration and motility, and its own authors said the result needed larger trials behind it, while our top grade is reserved for several trials plus pooled analyses that agree. The pooled work does rank carnitine at or near the top for sperm motility, so the effect on semen numbers looks real. What it does not show is more pregnancies: where pregnancy was pooled across carnitine, selenium and CoQ10 together, the rate was 16.2 per cent on treatment against 11.2 per cent on placebo, which did not reach significance. We do not recommend it for energy, for heart health or for athletic performance, and each of those has its own reason. On energy, the cell carried no citation at all and the nearest thing to on-target evidence is a meta-analysis of fatigue treatments in cancer and stem-cell transplant patients, in which the treatments that worked were erythropoietin and methylphenidate rather than L-carnitine; our literature search found no trial of it for fatigue in healthy adults. On heart health, the pooled trials are in people who have had a heart attack, and they are null on death, heart failure, unstable angina and repeat heart attack alike; the one favourable finding is a heart-scan measure in heart failure, which is a marker rather than an outcome. Neither study behind our grades has an exercise endpoint, and the meta-analysis that does exist looked at muscle soreness and the blood markers of muscle damage after exercise rather than at performance itself. That meta-analysis records no measure of VO2 max, time trial, power or time to exhaustion. 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 fertility. 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
Last reviewed 14 September 2026. Each interaction below lists the evidence its assessment drew on: some rest on studies of the exact pair, others on mechanism evidence, where a citation may support one part of the reasoning rather than the exact combination. The evidence line on each entry says which.
Additive anticoagulation
Amber Does L-Carnitine interact with Acenocoumarol?
Two published case reports describe L-carnitine making acenocoumarol, a warfarin-type blood thinner, work more strongly. In the more recent one, from 2004, a man with a mechanical heart valve saw his INR rise to 4.65 while he took 1,000mg of L-carnitine a day, and it settled back into his usual range after he stopped. That is still thin evidence, but if you take acenocoumarol and use L-carnitine, mention it to your anticoagulant clinic and have your INR checked.
Evidence: Limited human data, from small studies or case reports.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does L-Carnitine interact with Warfarin sodium?
Two published case reports describe L-carnitine making acenocoumarol, a close relative of warfarin, work more strongly, and in the more recent one the INR settled once L-carnitine was stopped. Neither involved warfarin itself, so the evidence for warfarin is thin. If you take warfarin, mention L-carnitine to your anticoagulation clinic and ask whether an INR check is worth doing after you start.
Evidence: No direct study of this exact combination yet; the caution rests on a plausible mechanism or on reports involving a related medicine.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Reduced thyroid-hormone effect
Amber Does L-Carnitine interact with 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.
Evidence: Supported by human study data.
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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How we grade severity, choose what's in scope, and what we exclude.
Every call on this page is reasoned. We publish the full rubric for severity tiers, the medication inclusion logic, the evidence grades we accept, and what we deliberately leave out. About three thousand words. Worth reading once if you use this tool more than occasionally.
Read the full methodologyA full supplement review, built around you, not a generic stack.
The free checker answers one interaction question at a time. A Distil report reviews the bigger picture: your goals, diet, medications, current supplements and any blood results you provide, then shows what appears worth keeping, changing, adding or leaving out.
- what your diet already covers, so you are not adding a capsule for something food is already handling
- which evidence-graded compounds are worth considering for your goals, and which were considered but left out
- what to keep, change or stop from what you already take, plus the order to introduce anything new
- how your medications and complete supplement stack fit together, with daily timing and a clear point for reassessment
The interaction check is one section of the full report. You also get the reasoning behind each recommendation and the evidence references used.
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