# why is metronidazole prescribed for SIBO, even though it's only active against anaerobes?



## vanessa6801 (Apr 12, 2014)

Hello all, I have a few questions about the treatment of sibo which I hope that someone will be able to answer. 
Metronidazole only kills anaerobic bacteria, and protozoans. And my understanding is that SIBO is an overgrowth of both anaerobic and aerobic bacteria. I havent read anywhere that sibo is an overgrowth of anaerobes alone. So, im wondering why metronidazole is prescribed for SIBO when it is a narrow spectrum antibiotic? Wouldnt a broad spectrum antibiotic, like rifaximin, be more effective in treating sibo? Even after all the anaerobes have been killed by metronidazole, the remaining bacteria (the aerobes) could still constitute an overgrowth, right? 
If anyone here has tried both metronidazole and rifaximin for treating their sibo, please let me know your experience, which one was more effective in reducing symptoms?
I have taken a combination of metronidazole and roxithromycin (roxithromycin is not commonly prescribed for sibo,and there are no trials proving its efficacy so I have no idea why my doctor prescribed that) and metronidazole on its own for the treatment of sibo. My sibo symptoms have been reduced by 60% as a result of treatment with these antibiotics. I am curious about whether a different antibiotic would be more effective and help in reducing symptoms further.


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## Kathleen M. (Nov 16, 1999)

Because they used antibiotics prior to the invention of rifamixin being approved. There are several antibiotics used, especially for the more classic diagnosis of SIBO.

http://www.emedicinehealth.com/small_intestinal_bacterial_overgrowth/page8_em.htm lists the various ones used.

Some may also have to do with cost issues of rifaximin

goodrx.com (which is a good source for seeing if there are coupons or things you can use for reducing prescription costs.

Metronidazole (Flagyl) is in the $10-15 range for 14 tablets

Rifamixin is $340-440 for 14 tablets.


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## vanessa6801 (Apr 12, 2014)

I have done some googling and have found out that over 99% of gut bacteria are anaerobes. That would explain why metronidazole is effective for SIBO, even though it is only active against anaerobes. 
However, it seems that aerobes can cause some problems too:

The definition of SIBO in culturing studies is 105 colony-forming units per milliliter of proximal jejunal aspiration.23 The bacteria that are most commonly overgrown are anaerobes (Bacteroides in 39%,Lactobacillus in 25%, and Clostridium in 20%) and aerobes (Streptococcus in 60%, Escherichia coli in 36%,Staphylococcus in 13%, and Klebsiella in 11%).23 In another study,20 the findings showed E coli in 37%,Enterococcus species in 32%, Klebsiella pneumoniae in 24%, and Proteus mirabilis in 6.5%.

I am hopeful that trying a different antibiotic - rifaximin- would do some good-- because it may be able to kill strains of bacteria that metronidazole wasnt able to kill.


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