Vitamin C and medications.
Vitamin C is in the Distil supplement database, evidence Grade A. The page below lists every medication we have explicitly assessed it against.
Vitamin C is a cofactor for collagen synthesis and an antioxidant that supports immune function, skin, and wound healing, and it has one particularly useful interaction: it markedly improves the absorption of non-haem iron from plant foods, which helps vegetarians and vegans. The everyday dose is 500 to 1,000mg, and splitting it across the day absorbs better than one large hit. Smokers need a little extra because smoking depletes it. If a skin or collagen goal is the point, take it alongside collagen peptides, since the body cannot build collagen without it; it also works in an antioxidant network with quercetin. The limits are about dose and kidneys. Above 2g a day it can raise kidney stone risk in predisposed people, and in established kidney disease high doses are best avoided, with intake capped around 500mg. The main side effect is loose stools above a gram or so. For most people, a modest split dose with food covers the benefit comfortably.
Below are the 8 documented pairs we have explicitly assessed for Vitamin C: 3 amber and 5 green. The pairs cluster around 4 mechanisms: Reduced anticoagulant effect, Reduced immunosuppressant level, Absorption interference, and Beneficial combination. 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
Reduced anticoagulant effect
Very high doses of vitamin C have been linked in rare reports to a reduced effect of vitamin-K-based blood thinners like acenocoumarol, which could lower your INR. Normal vitamin C intake is not a concern. If you take acenocoumarol and use high-dose vitamin C, keep it steady and mention it to your anticoagulant clinic.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
There are reports of large daily doses of vitamin C making warfarin work less well, so your INR (the blood test that shows how thinned your blood is) drops below the level you need. In each case the INR came back up once the vitamin C was stopped. A normal dietary amount or a small daily supplement is not the concern here. If you take a high-dose vitamin C supplement, tell whoever manages your warfarin so your INR can be watched, and avoid starting or stopping it without letting them know.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Reduced immunosuppressant level
There is a report that vitamin C, taken together with vitamin E, lowered ciclosporin blood levels in heart transplant patients by around a third. Ciclosporin has to stay within a tight range to stop the body rejecting a transplant, so a drop like that matters. If you have a transplant or take ciclosporin for another reason, do not start a vitamin C supplement without talking to your transplant team, who can check your ciclosporin level if anything changes. The same report found no effect on tacrolimus.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Absorption interference
This is worth knowing rather than a problem. Vitamin C is less stable when the stomach is less acidic, and acid-lowering medicines like esomeprazole modestly reduce the amount of vitamin C in your blood. The drop is small, it is the medicine affecting the vitamin rather than the other way round, and it does not stop you taking a vitamin C supplement. If anything, people on long-term acid-lowering treatment are the ones who may benefit most from keeping a good vitamin C intake.
This is worth knowing rather than a problem. Vitamin C is less stable when the stomach is less acidic, and acid-lowering medicines like lansoprazole modestly reduce the amount of vitamin C in your blood. The drop is small, it is the medicine affecting the vitamin rather than the other way round, and it does not stop you taking a vitamin C supplement. If anything, people on long-term acid-lowering treatment are the ones who may benefit most from keeping a good vitamin C intake.
This is worth knowing rather than a problem. Vitamin C is less stable when the stomach is less acidic, and acid-lowering medicines like omeprazole modestly reduce the amount of vitamin C in your blood. The drop is small, it is the medicine affecting the vitamin rather than the other way round, and it does not stop you taking a vitamin C supplement. If anything, people on long-term acid-lowering treatment are the ones who may benefit most from keeping a good vitamin C intake.
This is worth knowing rather than a problem. Vitamin C is less stable when the stomach is less acidic, and acid-lowering medicines like pantoprazole modestly reduce the amount of vitamin C in your blood. The drop is small, it is the medicine affecting the vitamin rather than the other way round, and it does not stop you taking a vitamin C supplement. If anything, people on long-term acid-lowering treatment are the ones who may benefit most from keeping a good vitamin C intake.
Beneficial combination
This can be a helpful pairing rather than a problem, especially if you have a stomach condition that lowers your stomach acid. Levothyroxine is absorbed better when the stomach is acidic, and taking it with vitamin C improved thyroid blood results in people whose levels had been hard to control. For most people with a normal stomach this makes little difference, and levothyroxine is still best taken on an empty stomach. If your thyroid levels have been difficult to settle, it is worth raising vitamin C with your prescriber rather than changing anything on your own.
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.
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 methodologyWant this checked across everything you take?
This page checks the pairs you enter. The personalised Distil report goes further:
- the same graded, cited interaction check across your whole stack, not just the pairs you thought to type in
- where your current routine may be leaving you short of your goals
- the evidence-backed compounds worth adding, and the ones worth dropping
It's a paid report: £79, or £49 for the first 25 customers. The interactions check is one section of it, and you can read a real one in full before you buy.
See a real sample reportSomething missing?
If a supplement or medication you take isn't in our autocomplete, tell us. We go through what people flag every week and add what's missing.