When your doctor may recommend them

• Vitamin D: deficiency is common in Oman despite the sun; it is checked and replaced at the dose your doctor sets.
• Calcium with vitamin D during steroid treatment, to protect the bones.
• Iron: only when deficiency is proven. Taking it every other day may be gentler on the stomach, and your doctor may prefer intravenous iron in active inflammatory bowel disease.
• Vitamin B12: regular injections with disease or surgery of the last part of the small bowel, or when deficiency is proven.
• Folic acid: with methotrexate or sulfasalazine, and before and during pregnancy.
• Zinc: when deficiency is proven, especially with chronic diarrhoea or fistulas, and for a limited time; long-term high doses cause copper deficiency and anaemia.
• Multivitamins: after bariatric surgery, and useful with restrictive diets such as keto or very low intake on weight-loss injections.
• Magnesium oxide for chronic constipation; not with kidney disease without advice, and take it at least 2 hours apart from thyroid tablets, iron and some antibiotics.

Good to know

• Omega-3 (fish oil): lowers blood triglycerides, but did not prevent Crohn's disease relapse in large trials. Eating fish twice a week is better than capsules.

• Vitamin D for IBS: study results are mixed; take it only to correct a proven deficiency.
• Vitamin E for fatty liver: on prescription only, for selected patients.
• Vitamin K1: prescribed when deficient because of poor fat absorption or liver disease. Vitamin K2 is a food supplement with no evidence of benefit in digestive diseases; do not take it with warfarin without medical advice, as it weakens warfarin's effect.

Not recommended

• Vitamin K3 (menadione): not used in humans because it is toxic; it can break down red blood cells (especially in G6PD deficiency) and damage the liver.
• High doses of vitamin D or vitamin A without testing and follow-up; they can cause toxicity.
• Iron tablets without testing; they can worsen gut symptoms and delay finding the cause of anaemia.
• "Immune boosters" and multi-ingredient mixtures, especially with immune-suppressing medicines or biologics.