What the FDA labels say about taking them together
Phenobarbital and sapropterin
Sapropterin's FDA label of August 26, 2024 names phenobarbital in its drug interactions section; the phenobarbital labels PlainMeds reads (2) do not name sapropterin. The sapropterin label: “Inhibitors of Folate Synthesis Clinical Impact In vitro and in vivo nonclinical data suggest that drugs that inhibit folate synthesis may decrease the bioavailability of endogenous BH4 by inhibiting the enzyme …”
Check it at the source: the phenobarbital FDA label on DailyMed (effective August 27, 2026); the sapropterin FDA label on DailyMed (effective August 26, 2024, BioMarin Pharmaceutical Inc.).
What does the phenobarbital label say about sapropterin?
Phenobarbital's label (effective August 27, 2026) and its 1 other brand-application label do not name sapropterin in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Phenobarbital), Chartwell RX, LLC, effective August 27, 2026 (SPL set 74428144-fb24-40a5-a670-3b876e2d116c), via openFDA; the full label on DailyMed
What does the sapropterin label say about phenobarbital?
Sapropterin label, drug interactionsInhibitors of Folate Synthesis (e.g., methotrexate, valproic acid, phenobarbital, trimethoprim) Clinical Impact In vitro and in vivo nonclinical data suggest that drugs that inhibit folate synthesis may decrease the bioavailability of endogenous BH4 by inhibiting the enzyme dihydrofolate reductase, which is involved in the recycling (regeneration) of BH4. This reduction in net BH4 levels may increase Phe levels. Intervention Consider monitoring blood Phe levels more frequently during concomitant administration. An increased dosage of KUVAN may be necessary to achieve a biochemical response.
Source: FDA drug label (Kuvan), BioMarin Pharmaceutical Inc., effective August 26, 2024 (SPL set af38711e-8873-4790-a92d-4d583e23fb89), 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
| Phenobarbital | Sapropterin | |
|---|---|---|
| Labeled for | Seizures and epilepsy | n/a |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $0.213 per tablet 100 mg tablet | n/a |
| Shortage now | A presentation being discontinued | 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