Question Regarding Doxycycline - Magnesium Interaction

In our latest question and answer, the pharmacist discusses the interaction between magnesium and doxycyline.


Why not use magnesium with doxycycline treatment for bacterial infections?

Asked by Janie On May 31, 2024

Answered by
Medical Content Reviewed By HelloPharmacist Staff

Published May 31, 2024
Last updated Jun 14, 2024


Thanks for your question!

I assume you are referring to our herbal-drug interaction checker when you ask why no interaction is showing between magnesium and doxycycline. I double-checked, and our checker does show an interaction. It states:

Magnesium decreases the absorption of tetracyclines. Magnesium can form insoluble complexes with tetracyclines in the gut and decrease their absorption and antibacterial activity. Advise patients to take these drugs 1 hour before or 2 hours after magnesium supplements.
HelloPharmacist Herbal Interaction Checker

If you are seeing conflicting information anywhere, please let us know so we can look into it.

More Discussion on Interaction

Although studies vary on just how much magnesium can affect tetracycline absorption (doxycycline is considered a tetracycline), it is well known that it can have an impact. The mechanism behind this is that tetracycline drugs bind to magnesium ions in the gastrointestinal tract, forming a complex that cannot be absorbed into your system.

Again, the results of studies vary, but one study showed that when tetracycline (250 mg) was taken together with 30 mL of aluminum-magnesium hydroxide gel, its oral bioavailability (rate and extent of absorption) dropped by 90%. This decrease in bioavailability could significantly affect how effective the antibiotic is in treating an infection.

To avoid this interaction, be sure to separate the administration of magnesium-containing products with tetracycline drugs by at least a few hours.

Final Words

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  • Effect of cimetidine and antacids on gastrointestinal absorption of tetracycline, PubMed

About the Pharmacist

Dr. Brian Staiger, PharmD

Dr. Brian has been practicing pharmacy for over 13 years and has wide-ranging experiences in many different areas of the profession. From retail, clinical, program development, and administrative responsibilities, he's your knowledgeable and go-to source for all your pharmacy and medication-related questions! Dr. Brian Staiger also has herbalist training and educational certificates in the field of medical ethnobotany. Feel free to send him an email at [email protected]! You can also connect with Dr. Brian Staiger on LinkedIn.

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