Vitamin D3 and medications.
Vitamin D3 is classified as a foundation supplement in the Distil database, evidence Grade A. The page below lists every medication we have explicitly assessed it against.
Vitamin D3 is the form your skin makes from sunlight, and at UK latitudes there is not enough sun between October and April to make it, which is why deficiency is so common, especially in people with darker skin, indoor workers, and anyone over 50. It supports bone density, immune function, mood, and calcium regulation, and the evidence here is strong: a 25-trial meta-analysis found supplementation reduced respiratory infections. The honest target is a blood level of 100 to 150 nmol/L, so testing beats guessing the dose. Two interactions matter most. Magnesium is needed to convert D3 into its active form, so the two tend to work best taken together. Vitamin K2 helps direct calcium to bone rather than arteries, which is why D3 is rarely recommended alone. Thiazide diuretics raise calcium and warrant a word with your GP. Take it with food, and retest after a few months rather than dosing blind.
Below are the 10 documented pairs we have explicitly assessed for Vitamin D3: 6 amber and 4 green. The pairs cluster around 4 mechanisms: Absorption interference, Drug depletes the supplement, Hypercalcaemia risk, 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
Absorption interference
Orlistat works by blocking the gut from absorbing some dietary fat, and because vitamin D is fat-soluble it gets absorbed less well too. This may lower your vitamin D level over time, even if you take a supplement at the same time of day. The usual advice is to separate them: take your vitamin D (or a fat-soluble multivitamin) at bedtime, at least 2 hours away from any orlistat dose, and your prescriber may check your vitamin D level.
Drug depletes the supplement
Carbamazepine speeds up the liver enzymes that break vitamin D down, so people on it for a long time tend to have lower vitamin D levels and, over years, a higher risk of weaker bones. This works the opposite way to most interactions: the medicine lowers your vitamin D rather than the supplement affecting the medicine. Taking a vitamin D supplement is generally the recommended response, and many prescribers check vitamin D and bone health in people on long-term carbamazepine.
Phenytoin speeds up the breakdown of vitamin D in the body, so people on long-term phenytoin often have lower vitamin D levels and, over years, can lose bone density. This is a reason to take vitamin D rather than avoid it: many specialists check vitamin D and supplement people on long-term phenytoin. It is worth letting your prescriber know so your vitamin D level and bone health can be kept under review and the dose set for you.
Hypercalcaemia risk
Thiazide diuretics like bendroflumethiazide make the kidneys hold on to more calcium, and vitamin D raises how much calcium you absorb from food. Taken together, especially at higher vitamin D doses or alongside calcium supplements, the two may push blood calcium too high. Standard replacement doses of vitamin D with a routine calcium check tend to be fine, but it is worth your prescriber knowing you take both.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
High-dose vitamin D can raise blood calcium, and a high calcium level makes the heart more sensitive to digoxin, which already has a narrow safe range. Standard replacement doses of vitamin D with normal calcium are not expected to be a problem, but high-dose vitamin D, or vitamin D plus calcium supplements, is worth discussing with your prescriber so your calcium and digoxin can be kept in check.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Indapamide is a thiazide-like diuretic that makes the kidneys keep more calcium, and vitamin D increases how much calcium you absorb. Together, particularly at higher vitamin D doses or with calcium supplements, blood calcium may rise too high. Normal replacement doses with an occasional calcium check are usually fine, but let your prescriber know you take both.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Beneficial combination
These are expected to be taken together. Adequate vitamin D is recommended alongside a bisphosphonate like alendronic acid: it helps keep your blood calcium in a healthy range and supports the medicine working as intended. The one practical point is timing, not safety. Alendronic acid must be taken first thing on an empty stomach with plain water, staying upright for 30 minutes, so take any vitamin D product (especially a combined vitamin D plus calcium supplement) later in the day. If your vitamin D is very low, your GP may want to correct it before or as you start, so ask them to confirm your levels.
This is usually a helpful pairing rather than a problem when methylprednisolone is taken by mouth or injection for three months or more. Long-term steroids weaken bone and reduce how well you absorb calcium, so vitamin D (often with calcium) is routinely recommended alongside them to protect your bones. Short courses and one-off injections do not carry the same concern. At very high steroid doses the bone-protecting effect of vitamin D may be partly blunted, so if you are on long-term or high-dose steroids your prescriber may also check your bone health and consider additional bone medication.
This is usually a helpful pairing rather than a problem. Long-term steroids like prednisolone weaken bone and reduce how well you absorb calcium, so vitamin D (often with calcium) is routinely recommended alongside them to protect your bones. Guidelines advise keeping vitamin D in a healthy range for anyone taking steroids for three months or more. At very high steroid doses the bone-protecting effect of vitamin D may be partly blunted, so if you are on long-term or high-dose steroids your prescriber may also check your bone health and consider additional bone medication.
These two are commonly and deliberately taken together. Risedronate works best when your vitamin D level is in a healthy range, and keeping vitamin D topped up lowers the small chance of your blood calcium dropping after a dose. This is a helpful pairing, not a clash. Take risedronate first thing on an empty stomach with plain water and stay upright for 30 minutes; you can take your vitamin D later in the day.
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.