Drug interactions
Phentermine + Phenylephrine
Severity: Moderate
Phentermine and Phenylephrine: 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
Phentermine is a sympathomimetic amine that raises blood pressure and heart rate, and phenylephrine has pressor activity as well. Together they can cause additive hypertension, tachycardia and palpitations, which is relevant in patients with cardiovascular disease.
What this interaction means in plain words
Phentermine and Phenylephrine 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.
Phentermine belongs to Central Nervous System; Phenylephrine belongs to Respiratory.
Why it happens
Additive sympathomimetic effects, direct alpha-agonism from phenylephrine plus indirect norepinephrine release from phentermine.
What to do
Avoid decongestant doses of phenylephrine in patients on phentermine where possible. If used together, monitor blood pressure and heart rate.
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 Phenylephrine
- Isocarboxazid + Phenylephrine Contraindicated
- Phenelzine + Phenylephrine Contraindicated
- Phenylephrine + Tranylcypromine Contraindicated
- Bromocriptine + Phenylephrine Major
- Cocaine + Phenylephrine Major
- Dihydroergotamine intranasal + Phenylephrine Major
Other interactions of Phentermine
- Isocarboxazid + Phentermine Contraindicated
- Linezolid + Phentermine Contraindicated
- Phenelzine + Phentermine Contraindicated
- Phentermine + Procarbazine Contraindicated
- Phentermine + Rasagiline Contraindicated
- Phentermine + Selegiline Contraindicated
Frequently asked questions
Can I take Phentermine with Phenylephrine?
MediHelp rates the interaction as moderate. Phentermine and Phenylephrine 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. Phentermine is a sympathomimetic amine that raises blood pressure and heart rate, and phenylephrine has pressor activity as well. Together they can cause additive hypertension, tachycardia and palpitations, which is relevant in patients with cardiovascular disease.
How serious is the interaction between Phentermine and Phenylephrine?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Avoid decongestant doses of phenylephrine in patients on phentermine where possible. If used together, monitor blood pressure and heart rate.
What other medicines interact with Phenylephrine?
MediHelp also lists interactions of Phenylephrine with, for example, Isocarboxazid, Phenelzine, Tranylcypromine. The full list is on the Phenylephrine page.
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