Narrow-Range Medicines and Supplements: Warfarin, Digoxin, Lithium, Cyclosporine
Why warfarin, digoxin, lithium and cyclosporine need extra care with herbs, supplements and some foods, and what official sources say about St. John's wort, NSAIDs and grapefruit.
Content last updated . This page has not been medically reviewed.
Ask before adding anything
If you take warfarin, digoxin, lithium or cyclosporine, ask the prescriber or pharmacist before adding any herb, supplement, vitamin or new over-the-counter product, and before changing your diet in a big way. This page does not tell anyone to stop or change a prescribed medicine. For collapse, a seizure or trouble breathing, call 911. For possible overdose or poisoning, Poison Help is 1-800-222-1222 (US). For a mental health crisis, call or text 988 (US).What "narrow range" means
A narrow therapeutic index medicine is one with a small margin between a helpful level in the body and a harmful one. [1] For these medicines a small rise or fall in the level can matter. The NHS makes the same point for lithium: some medicines can change the levels of lithium in your blood. [2] This page covers the four medicines in the title. A pharmacist can say whether others you take are in this group.What sources document
Labels: documented (an official source states it), reported (case reports), preliminary, theoretical. See how interactions work.| Medicine | Supplement or herb | What the source says | Label |
|---|---|---|---|
| Warfarin | St. John's wort (Hypericum perforatum) | NCCIH lists warfarin among medicines whose effects St. John's wort can weaken. [3] The NHS says not to take St. John's wort or vitamin K supplements with warfarin. [4] | Documented |
| Warfarin | Ginkgo, garlic, ginseng, goldenseal, echinacea, coenzyme Q10 | MedlinePlus lists these among nonprescription or herbal products that may interact with warfarin. [5] | Documented caution |
| Warfarin | Glucosamine | NCCIH says glucosamine may increase the effects of warfarin, raising the risk of serious bruising and bleeding. [6] | Documented caution |
| Digoxin | St. John's wort | The NHS says St. John's wort can affect how digoxin works. [7] MedlinePlus lists St. John's wort and calcium supplements among products that may interact. [8] | Documented |
| Cyclosporine | St. John's wort | MedlinePlus lists St. John's wort as a herbal product that may interact. [9] FDA says medicines for organ transplants are less effective with St. John's wort. [10] | Documented |
| Lithium | Herbs and supplements in general | The NHS says there is not enough information to say complementary medicines, herbal remedies and supplements are safe with lithium. [2] | Information gap |
| Digoxin | Other herbs and supplements | The NHS says the same for other products with digoxin. [7] | Information gap |
For St. John's wort in detail, see St. John's wort interactions. NCCIH says interactions with it can weaken the effects of life-saving medicines or cause dangerous side effects. [3]
Common over-the-counter products
- NSAIDs (ibuprofen, naproxen). The NHS lists NSAIDs among medicines that may affect lithium levels. [2] MedlinePlus lists NSAIDs for lithium and for cyclosporine, and aspirin and NSAIDs for warfarin. [11] [9] [5] A 2026 case report described severe lithium toxicity after lithium and an NSAID, and says NSAIDs are recognized to raise lithium levels by reducing kidney clearance. [12] That is reported, supported by documented labeling.
- Antacids and calcium. MedlinePlus lists antacids and calcium supplements for digoxin. [8]
Food and drink
Foods high in vitamin K, such as broccoli, spinach and other green leafy vegetables, can affect how warfarin works, according to the NHS; MedlinePlus advises consistent amounts week to week and asks you to talk to your doctor before diet changes. [4] [5] The NHS also lists cranberry juice and products, grapefruit juice and large amounts of alcohol with warfarin. [4] MedlinePlus says to avoid grapefruit with cyclosporine, and for lithium points to a proper diet with the right amounts of fluid and salt. [9] [11] See food and drink interactions, leafy greens and vitamin K.Who should be especially careful
Bring a written list of everything you take, or the bottles, to the pharmacist and every prescriber. [13] See older adults and supplements before surgery.What we could not find
Tacrolimus and other transplant medicines: Proper evidence not found in the pages we read this session (the NHS tacrolimus page was not available to us). Specific herbs other than those above with lithium or digoxin: the NHS says there is not enough information. [2] Absence of reports is not safety.Frequently asked questions
Why do these medicines get special warnings?
Their margin between helpful and harmful levels is small. [1]
Is St. John's wort safe if my blood tests are normal?
No source we read says so. NCCIH says it can weaken important medicines including warfarin, cyclosporine and digoxin. [3]
Can I take ibuprofen with lithium?
Sources list NSAIDs as products that can raise lithium levels. [2] [11] Ask the prescriber or pharmacist first.
Does a vitamin count?
Yes. The NHS names vitamin K supplements with warfarin. [4] Bring every vitamin to the pharmacist.
What should I bring to appointments?
A written list of everything you take, or the bottles themselves. [13] See how to check an interaction safely.
References
- PubMed, Narrow Therapeutic Index Drugs: Regulatory Frameworks, Bioequivalence, and Model-Informed Precision Dosing (review, 2026). Read 2026-10-22.
- NHS, Taking lithium with other medicines and herbal supplements (NHS, page last reviewed 9 August 2023). Read 2026-10-22.
- NCCIH, St. John’s Wort: Usefulness and Safety (NCCIH, updated May 2025). Read 2026-10-22.
- NHS, Warfarin (NHS, page last reviewed 24 February 2026). Read 2026-10-22.
- MedlinePlus, Warfarin: MedlinePlus Drug Information (last revised 15 March 2026). Read 2026-10-22.
- NCCIH, Know the Science: How Medications and Supplements Can Interact, Some Supplements May Increase the Effects (NCCIH). Read 2026-10-22.
- NHS, Taking digoxin with other medicines and herbal supplements (NHS, page last reviewed 22 March 2023). Read 2026-10-22.
- MedlinePlus, Digoxin: MedlinePlus Drug Information (last revised 15 June 2017). Read 2026-10-22.
- MedlinePlus, Cyclosporine: MedlinePlus Drug Information (last revised 15 November 2023). Read 2026-10-22.
- FDA, Mixing Medications and Dietary Supplements Can Endanger Your Health (FDA Consumer Update, content current 2 June 2022). Read 2026-10-22.
- MedlinePlus, Lithium: MedlinePlus Drug Information (last revised 15 April 2017). Read 2026-10-22.
- PubMed, Severe Lithium Toxicity Following Concurrent Lithium and Nabumetone Use in a Patient With Bipolar I Disorder: A Case Report (2026). Read 2026-10-22.
- NCCIH, Know the Science: How Medications and Supplements Can Interact, Talk With Your Health Care Providers (NCCIH). Read 2026-10-22.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.