Demeclocycline and Magnesium: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Demeclocycline
Magnesium
No brand names on record
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Reduced efficacy of tetracyclines
Interaction Deep Dive
Aluminum, calcium or magnesium containing products may impair the absorption of orally administered tetracyclines (up to 90%). If the patient is taking both medications, space them at least 1 to 2 hours apart2.
Why it happens (mechanism)
Reduced absorption and half-life of tetracyclines
Literature reports
3 reports — tap to read
a) Antacids may significantly enhance the total body clearance and renal clearance of intravenous doxycycline and may reduce oral bioavailability 3.
b) Antacids containing divalent or trivalent cations (calcium, magnesium, aluminum) impair the absorption of oral tetracyclines and result in decreased serum levels 14. Milk and milk products reportedly affect oral doxycycline absorption to a lesser degree than tetracycline 3.
c) Combined therapy with oral antacids (aluminum hydroxide gel) and intravenous doxycycline resulted in a significant increase in the total clearance and renal clearance of doxycycline for six healthy volunteers. The subjects were given intravenous doxycycline 200 mg on two occasions, separated by a 7-day washout period; on one occasion, oral aluminum hydroxide gel was given concomitantly beginning 48 hours prior to the doxycycline and continuing for a total of 16 doses as 30 mL four times daily. When given concomitantly with antacids, the half-life of doxycycline decreased from 16.2 to 11.2 hours and the total body clearance increased from 37.4 to 54.1 mL/min. In addition, the area under the plasma concentration-time curve (AUC) decreased significantly with an increase in renal clearance 5.
Common questions
Can I take Demeclocycline and Magnesium together?
Reduced efficacy of tetracyclines Always confirm with your pharmacist or prescriber before making any change.
How serious is the Demeclocycline and Magnesium interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.
How strong is the evidence for this interaction?
The evidence is graded "established". Well documented — supported by controlled studies or strong clinical data.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Demeclocycline or Magnesium need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (5)
- Neuvonen PJ: Interactions with the absorption of tetracyclines. Drugs 1976b; 11:45-54. PubMed
- D'Arcy PF & McElnay JC: Drug-antacid interactions: assessment of clinical importance. Drug Intell Clin Pharm 1987; 21:607-617. PubMed
- Product Information: Vibramycin(R), doxycycline. Pfizer Laboratories, New York, NY, 1990. DailyMed
- Chin TF & Lach JL: Drug diffusion and bioavailability: tetracycline metallic chelation. Am J Hosp Pharm 1975; 32:625-629. DOI
- Nguyen VX, Nix DE, Gillikin S, et al: Effect of oral antacid administration on the pharmacokinetics of intravenous doxycycline. Antimicrob Agents Chemother 1989; 33:434-436. PubMed
Keep reading about Demeclocycline
Keep reading about Magnesium
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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