Several common supplements interact meaningfully with medicines. Examples include vitamin K affecting blood thinners such as warfarin, St John's wort reducing the effect of many drugs including some antidepressants and contraceptives, calcium or iron reducing the absorption of certain antibiotics and thyroid medicine, and high-dose fish oil or vitamin E adding to bleeding risk with anticoagulants.
Because supplements are sold without prescription, people often assume they are harmless alongside medication, but timing and dose can matter a lot. Sometimes separating a supplement and a medicine by a few hours solves the problem; other combinations should be avoided entirely.
Keep a current list of everything you take, including over-the-counter products and herbal remedies, and review it with a pharmacist or doctor, especially before starting anything new or around surgery. If you notice new symptoms after combining a supplement with medication, seek advice promptly.