Drug interactions
Fosphenytoin + Metformin
Severity: Moderate
Fosphenytoin and Metformin: moderate interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Phenytoin, the active form of fosphenytoin, can impair insulin secretion and raise blood glucose. This may reduce the glucose-lowering effect of metformin and worsen glycemic control in people with diabetes.
What this interaction means in plain words
Fosphenytoin and Metformin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Fosphenytoin belongs to Central Nervous System; Metformin belongs to Endocrine.
Why it happens
Phenytoin inhibits pancreatic insulin release, which opposes the antihyperglycemic action of metformin (pharmacodynamic antagonism).
What to do
Monitor blood glucose when fosphenytoin is started, changed or stopped, and adjust the metformin dose as needed. Watch for hypoglycemia if fosphenytoin is withdrawn.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Metformin
Other interactions of Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Frequently asked questions
Can I take Fosphenytoin with Metformin?
MediHelp rates the interaction as moderate. Fosphenytoin and Metformin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Phenytoin, the active form of fosphenytoin, can impair insulin secretion and raise blood glucose. This may reduce the glucose-lowering effect of metformin and worsen glycemic control in people with diabetes.
How serious is the interaction between Fosphenytoin and Metformin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor blood glucose when fosphenytoin is started, changed or stopped, and adjust the metformin dose as needed. Watch for hypoglycemia if fosphenytoin is withdrawn.
What other medicines interact with Metformin?
MediHelp also lists interactions of Metformin with, for example, Contrast media, Diatrizoate meglumine / diatrizoate, Diatrizoate. The full list is on the Metformin page.
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