Supplement safety

Supplements can correct a deficiency, but more is not better. Several popular bone and joint supplements have upper limits, interact with medicines, or caused harm at high doses in trials. This page collects the main risks in one place.

Upper limits

Supplement	Adult upper limit	Risk above it
Calcium (all sources)	2,500 mg (19 to 50), 2,000 mg (51 and over)	Kidney stones, possible other harms1
Magnesium (supplements only)	350 mg	Diarrhoea, stomach upset; dangerous in kidney disease2
Vitamin K	None set	Interferes with warfarin at any dose3

When high doses backfired

• Very high vitamin D. In a trial of 2,256 women aged 70 and over, a single yearly dose of 500,000 IU of vitamin D led to more falls and fractures than placebo.⁴
• Calcium with vitamin D. In the Women’s Health Initiative, daily calcium and vitamin D raised the risk of kidney stones by 17% while hip fracture reduction was small and uncertain.⁵

Interactions to know

• Warfarin: vitamin K counteracts it, and glucosamine has been reported to increase its effect. Discuss any change with your clinician.
• Kidney disease: avoid magnesium and calcium supplements unless prescribed.
• Antibiotics and thyroid medicine: calcium and magnesium can reduce their absorption; take them apart.
• Shellfish allergy: many glucosamine products are made from shellfish.

Questions to ask first

1. Am I actually low? A blood test or a dietary check can tell.
2. Can I get it from food instead?
3. What dose, and will the total from food and supplements stay under the upper limit?
4. Does it interact with anything I take?

Supplements and back pain

No supplement has good evidence for treating common back pain. Vitamin D helps only if you are deficient; see vitamin D. Glucosamine did not beat placebo; see glucosamine and chondroitin. What works is covered in what works for back pain.

Footnotes

1. Calcium: fact sheet for health professionals (NIH Office of Dietary Supplements). 2024. ↩

2. Magnesium: fact sheet for health professionals (NIH Office of Dietary Supplements). 2022. ↩

3. Vitamin K: fact sheet for health professionals (NIH Office of Dietary Supplements). 2021. ↩

4. Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial. 2010. ↩

5. Calcium plus vitamin D supplementation and the risk of fractures (Women’s Health Initiative). 2006. ↩

6. Supplements for bone health (Archives of Endocrinology and Metabolism). 2026.