Ferrous sulfate Doxycycline

The oral dosage for rapidly excreted tetracyclines, equivalent to tetracycline hydrochloride, is 0.25-0.5 g four times daily for adults and 20-40 mg/kg/d for children (8 years of age and older). For severe systemic infections, the higher dosage is indicated, at least for the first few days. The daily dose is 600 mg for demeclocycline or methacycline, 100 mg once or twice daily for doxycycline, and 100 mg twice daily for minocycline. Doxycycline is the oral tetracycline of choice because it can be given as a once-daily dose and its absorption is not significantly affected by food. All tetracyclines chelate with metals, and none should be orally administered with milk, antacids, or ferrous sulfate. To avoid deposition in growing bones or teeth, tetracyclines should be avoided in pregnant women and children less than 8 years of age. 

A 46-year-old woman is prescribed doxycycline for a diagnosis of community-acquired pneumonia. Her PMH is significant for iron-deficiency anemia, peptic ulcer disease, HTN, a recent DVT, and headaches. Her current medications include ferrous sulfate, ibuprofen, enalapril, acetaminophen, famotidine, and warfarin. Which of her following medications is most likely to result in decreased levels of doxycycline ... 

An investigation in 10 healthy subjeets given single oral doses of tetracy-elines showed that ferrous sulfate 200 mg decreased the serum antibacterial levels as follows doxycycline 200 mg, 80 to 90% methacycline 300 mg, 80 to 85% oxytetracycline 500 mg, 50 to 60% and tetracycline 500 mg, 40 to 50%. Another study in 2 groups of 8 healthy subjects found that ferrous sulfate 300 mg reduced the absorption of tetracycline and minocycline by 81% and 77%, respectively. ... 

Other studies found that in some instances iron caused the tetracycline serum levels to fall below minimum bacterial inhibitory concentrations. If the iron was given 3 hours before or 2 hours after most tetracyclines the serum levels were not significantly reduced. " However, even when the iron was given up to 11 hours after doxycycline, serum concentrations were still lowered by 20 to 45%. In contrast to this, another study found that four doses of ferrous sulfate (each equivalent to 80 mg of elemental iron) starting 11.5 hours after doxycycline did not affect the absorption of a 200-mg dose of doxycycline, and only reduced the AUC of a 100-mg dose ofdoxycycline by 17%. ... 

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