What the FDA labels say about taking them together
Lithium and phenytoin
Lithium's FDA label of December 23, 2025 names phenytoin in its drug interactions section; the phenytoin labels PlainMeds reads (2) do not name lithium. The lithium label: “Concomitant use can lower serum lithium concentrations by increasing urinary lithium excretion.”
Check it at the source: the lithium FDA label on DailyMed (effective December 23, 2025, Glenmark Pharmaceuticals Inc., USA); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the lithium label say about phenytoin?
Lithium label, drug interactionsClinical Impact: Concomitant use can lower serum lithium concentrations by increasing urinary lithium excretion. Intervention: More frequent serum lithium concentration monitoring. Increase lithium dosage based on serum lithium concentration and clinical response [see Dosage and Administration ( 2.3 )]. Methyldopa, Phenytoin and Carbamazepine
Source: FDA drug label (Lithium Carbonate), Glenmark Pharmaceuticals Inc., USA, effective December 23, 2025 (SPL set 63b3790f-a9b5-47f4-bae1-ddf8b43c13c1), via openFDA; the full label on DailyMed
What does the phenytoin label say about lithium?
Phenytoin's label (effective October 15, 2024) and its 1 other brand-application label do not name lithium in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Phenytek), Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Lithium | Phenytoin | |
|---|---|---|
| Labeled for | Bipolar disorder | Seizures and epilepsy |
| Boxed warning | Yes | No |
| Pharmacy cost (NADAC) | $0.104 per capsule 300 mg capsule | $0.145 per capsule 100 mg capsule |
| Shortage now | No current entry | No current entry |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once