Drug interactions
Diphenhydramine + Phentermine
Severity: Minor
Diphenhydramine and Phentermine: minor 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 stimulant and diphenhydramine is sedating, so their effects on alertness partly oppose each other with an unpredictable net result. Additive anticholinergic and sympathomimetic effects such as tachycardia may also occur.
What this interaction means in plain words
Diphenhydramine and Phentermine have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised.
Diphenhydramine belongs to Respiratory; Phentermine belongs to Central Nervous System.
Why it happens
Opposing CNS effects; no established pharmacokinetic interaction.
What to do
No routine adjustment is required. Monitor heart rate and blood pressure in patients with cardiovascular disease.
What to ask your pharmacist
- Is there anything I should notice while taking both?
- Do I need to tell my doctor at the next visit?
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Other interactions of Diphenhydramine
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 Diphenhydramine with Phentermine?
MediHelp rates the interaction as minor. Diphenhydramine and Phentermine have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised. Phentermine is a sympathomimetic stimulant and diphenhydramine is sedating, so their effects on alertness partly oppose each other with an unpredictable net result. Additive anticholinergic and sympathomimetic effects such as tachycardia may also occur.
How serious is the interaction between Diphenhydramine and Phentermine?
MediHelp rates it minor, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: No routine adjustment is required. Monitor heart rate and blood pressure in patients with cardiovascular disease.
What other medicines interact with Diphenhydramine?
MediHelp also lists interactions of Diphenhydramine with, for example, Alfentanil, Belladonna and opium, Benzhydrocodone / acetaminophen. The full list is on the Diphenhydramine page.
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