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Gastric Bypass Reduces Bioavailability of Azithromycin

Bariatric surgery can profoundly affect how the body absorbs medications – this issue, however, remains largely understudied.

In a paper, just published in the Journal of Antimicrobial Chemotherapy, we examine the effect of gastric bypass surgery, a procedure that circumvents the upper gut on the bioavailability (absorption) of azithromycin, a widely used treatment for community-acquired infections.

We performed single-dose pharmacokinetic studies in 14 female post-gastric bypass patients and 14 sex- and body mass index (BMI)-matched controls (mean age 44 years and BMI 36.4).

Azithromycin concentrations, following the administration of two 250 mg tablets were about 30% lower in gastric bypass patients compared with controls.

This finding suggests that there may be a substantial risk for treatment failure with this antibiotic in and clinicians should consider dose modification and/or closer clinical monitoring of gastric bypass patients receiving azithromycin.

Calgary, Alberta

ResearchBlogging.orgPadwal RS, Ben-Eltriki M, Wang X, Langkaas LA, Sharma AM, Birch DW, Karmali S, & Brocks DR (2012). Effect of gastric bypass surgery on azithromycin oral bioavailability. The Journal of antimicrobial chemotherapy PMID: 22577100



  1. Mean age 44 years and BMI 36.4? Jesus. That pretty much describes me. I wear a 16W and many people wouldn’t even describe me as “fat.” Why on earth would someone my size get a gastric bypass? What kind of doctor would recommend that? So disturbing…

    I know that’s not the point of the article or the study, but isn’t that average BMI below the minimum allowable for that type of surgery?

    Oh wait. It’s allowable if they have a “comorbidity,” right? So if their blood pressure is on the high end of what used be considered normal 30 years ago or if they have “insulin resistance,” – which some ridiculous percentage of middle aged North Americans have regardless of size, and which isn’t even a disease – then they’re in.

    Shudder. But to each his own, I guess. You do what you have to do to escape discrimination. Cross your fingers and knock on wood that it doesn’t end with a lethal case of pneumonia, helped along by an immune system damaged by undernutrition and an inability to absorb antibiotics.

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    • That is probably after surgery the sizes they were. But that said I wore an 18 to 20 when I had bypass and without comorbidities,but my BMI was 40 at surgery.

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  2. Sorry DeeLeigh, I should have been clearer, these were weight stable patients who are now BMI 36 a year or more after their surgery. Many had BMIs of 50 or greater before their surgery.

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  3. Thanks for clarifying that. Could be equally problematic for folks who start out fatter, though…

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  4. Thank you for this information, Dr. Sharma. This would explain why I always have to have a “second round” of medication whenever I have been prescribed azithroycin for infection. Are there other studies that are currently looking at medications that might also affect bioavailability in gastric bypass patients? I hope that pharmaceutical companies will begin to research their medications and then publish dosing recommendations for gastric bypass patients; similar to how current dosing differs from adult to pediatric. With the increasing number of people having gastric bypass surgery, this becomes a vital issue, especially when medications are used for critical, life-threatening events.

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