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    Ingredient guide · Ingredient reference

    St Johns wort with magnesium: the full medicine list is the key safety question

    The inspected sources do not establish universal safety or a validated benefit from combining St Johns wort with magnesium. The herb interacts with many important medicines, and adding magnesium does not remove those interactions. Review the complete regimen before starting the combination.

    Read the evidence ↓

    Supplement Science Insights Editorial Team

    Published September 23, 2026

    Do not mistake a missing pairwise warning for clearance

    A short nutrient or herb fact sheet is not a comprehensive interaction screen. The inspected guidance does not establish a specific universal St Johns wort–magnesium dosing or spacing rule. The relevant decision includes all prescriptions, nonprescription medicines and supplement ingredients. [1][2]

    St Johns wort has clinically important interactions

    NCCIH warns that the herb can weaken effects of medicines including birth-control pills, warfarin, cyclosporine and some seizure, heart, HIV and cancer treatments. Combining it with certain serotonin-affecting medicines can also cause serious adverse effects. These concerns remain even if magnesium itself is tolerated. [1]

    Magnesium brings a different set of considerations

    Some antibiotics and osteoporosis medicines have absorption interactions with magnesium. Elemental dose and kidney function also matter. A schedule designed for a mineral absorption interaction cannot be assumed to solve an herbs systemic medicine interaction. [2][1]

    A shared mood claim does not prove added benefit

    NCCIH describes evidence for St Johns wort in some depression contexts, but that does not establish that adding magnesium makes it more effective or appropriate. Depression care and any medicine changes should be discussed with a clinician; pregnancy and breastfeeding require particular review. [1]

    Make the review concrete

    Imagine a person taking an antidepressant adds a magnesium product and then considers St Johns wort. The antidepressant–herb interaction may be the crucial issue, even though the original question named only the two supplements. Bring every label and prescription to the review. [1][2]

    Hypothetical example — review a hypothetical supplement pairing
    ItemReason to check
    St Johns wortMany important medicine interactions
    MagnesiumDose, kidney function and absorption interactions
    Other sleep or mood ingredientsMay change the risk
    Separate administration timesDo not guarantee compatibility
    St Johns wort with magnesium: the full medicine list is the key safety question — original article outline diagram
    Original editorial diagram of this article’s sections; no product photograph or clinical finding is depicted.

    Sources for this article

    1. St. John’s Wort: Usefulness and Safety | NCCIH
      Source consulted: 2026-09-21. This is a reference resource, not a claim that we conducted the studies or tested a product.
    2. Magnesium - Health Professional Fact Sheet
      Source consulted: 2026-09-21. This is a reference resource, not a claim that we conducted the studies or tested a product.