Drug interactions
Guanfacine + Phenytoin
Severity: Moderate
Guanfacine and Phenytoin: 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 lowers guanfacine exposure and shortens its half-life, which may weaken blood pressure or ADHD symptom control.
What this interaction means in plain words
Guanfacine and Phenytoin 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.
Guanfacine belongs to Cardiovascular; Phenytoin belongs to Central Nervous System.
Why it happens
CYP3A4 induction by phenytoin increases the metabolism of guanfacine.
What to do
Consider increasing the guanfacine dose, up to double for extended-release in ADHD, guided by response and blood pressure. Reduce the dose again if phenytoin is stopped.
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 Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Other interactions of Guanfacine
- Clonidine + Guanfacine Major
- Guanfacine + Lofexidine Major
- Aldesleukin + Guanfacine Moderate
- Amiodarone + Guanfacine Moderate
- Amisulpride + Guanfacine Moderate
- Amitriptyline + Guanfacine Moderate
Frequently asked questions
Can I take Guanfacine with Phenytoin?
MediHelp rates the interaction as moderate. Guanfacine and Phenytoin 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 lowers guanfacine exposure and shortens its half-life, which may weaken blood pressure or ADHD symptom control.
How serious is the interaction between Guanfacine and Phenytoin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Consider increasing the guanfacine dose, up to double for extended-release in ADHD, guided by response and blood pressure. Reduce the dose again if phenytoin is stopped.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
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