Can I take Levothyroxine 25 and Promethazine together?
Drug interaction guide
Originally published 25 Jan 2026
Meets Patient’s editorial guidelines
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There is no known direct interaction between levothyroxine and promethazine. However, if you have an overactive thyroid (hyperthyroidism), promethazine should be used with caution as it may slightly increase the risk of heart-related side effects. Since you are taking levothyroxine (usually for an underactive thyroid), this is generally only a concern if your dose is too high.
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Why this happens
Levothyroxine is a synthetic version of the thyroid hormone T4. Promethazine is an antihistamine with 'anticholinergic' properties. While they do not interfere with how each other is absorbed or broken down by the body, both can influence heart rate. In patients with excessive thyroid hormone levels, promethazine could potentially contribute to a fast or irregular heartbeat.
What you should do
It is generally safe to take these two medicines together. Ensure you take your levothyroxine at the same time every day (usually in the morning on an empty stomach). If you experience any heart palpitations, chest pain, or unusual restlessness while taking promethazine, contact your doctor. Always ensure your thyroid levels are being monitored regularly by your GP.
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Using other medicines
Taking multiple medicines? Our Medicines Interaction Checker helps you check whether your prescriptions, over-the-counter medicines, and supplements are safe to take together. Simply search for your medicines to see potential interactions and what to do about them.
Disclaimer
This information is for general educational purposes only and should not be relied upon as a substitute for professional medical advice. Always consult your GP, pharmacist, or another qualified healthcare professional before making decisions about your medications. Individual circumstances may vary, and only a healthcare professional who knows your medical history can provide personalised guidance.
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Article history
The information on this page is written and peer reviewed by qualified clinicians.
25 Jan 2026 | Originally published
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