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Before taking lactulose, tell your doctor and pharmacist what prescription and nonprescription medications you are taking, especially antacids, antibiotics including neomycin (Mycifradin), and other laxatives.” (Quote from here)

Why do antibiotics have a special importance when taking lactulose? Why is neomycin (Mycifradin) mentioned separately?

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@winerd-> Sorry, you're correct. Still think you should have merged the questions though, since you are quoting the same passage and asking the same general question on two substances (feels more like a 1a, 1b question than two separate). The same answerer can most likely answer both at once. –  fileunderwater Jun 16 '13 at 20:07
    
@fileunderwater: No problem, thanks. Let's see if the answers will be similar or not. (I thought they would talk about different mechanisms, but indeed I could be wrong.) –  winerd Jun 16 '13 at 20:29
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From the Wikipedia article (I'm sure there's a scholarly reference, but I think this is well-known):

Lactulose works by relieving constipation through a number of different methods. Lactulose is made up of sugar molecules and is partly broken down by the bacteria that live in the lower part of the gut.

So, certain antibiotics affect (and reduce) the intestinal flora (and neomycin especially so, which is I believe the reason why it is mainly used in ointments), leaving the lactulose undigested and not as effective in forcing the body to retain water in the colon, which is the mechanism through which the drug acts.

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Lactulose is an osmotic laxative. This means it works by drawing water into the colon (i.e. it retains a lot of water with it) and hence softens your stool in cases of constipation. As you can imagine, this interferes with absorption of most drugs including antibiotics and even food. Neomycin is especially toxic to gut flora (as mentioned above) and if taken with lactulose, it will linger in your GI tract for even longer and kill more gut flora. Your gut flora is important in protecting you against other harmful bacteria through competition for food, nutrients, etc. If you kill your flora, you are allowing other harmful organisms a field day in your intestine and they will grow and cause a condition known as pseudomembranous colitis (commonly caused by clostridium difficile infection). Also neomycin is mentioned separately because it has been associated with this condition which can sometimes be fatal (toxic megacolon, GI perforation, severe diarrhoea, etc)

In summary, you shouldn't be taking other medication whilst you're constipated anyway (especially when taking lactulose) because they won't get absorbed and may make the constipation worse.

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Is there any evidence that lactulose decreases absorption thereby leading to an increased intestinal concentration? –  AndroidPenguin Jun 18 '13 at 0:55
    
"Neomycin ... if taken with lactulose, it will linger in your GI tract for even longer" - Are you sure about this? I thought that any laxative would REDUCE the time any substance spends in the colon. –  winerd Jun 18 '13 at 12:35
    
@winerd but within the context of constipation, anything you take will linger in your GI tract for much longer. In theory, a laxative should increase your transit time but bearing in mind that lactulose interferes with absorption, this might be the case. –  John Jun 19 '13 at 7:18
    
@AndroidPenguin, this just makes sense to me-just like having a diarrhoea which also interferes with absorption. Most laxatives will eventually cause diarrhoea which will interfere with absorption of other nutrients. –  John Jun 19 '13 at 7:20
    
@John: I find it counter-intuitive that a laxative would increase transit time. Furthermore here it's stated for lactulose specifically and for many other laxatives that they "can cause SHORTER gastro intestinal transit time". Can you support your statement? –  winerd Jun 21 '13 at 16:30
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