I have been writing a lot about probiotics and health lately, but there is another side to the issue. Antibiotics are coming under increased scutiny as far as their impact on weight.
This study discusses how the rise of antibiotics correlates with the increase in obesity. This study ( full-text ) studies specific antibiotics and finds that they can lead to weight gain. With one particular antibiotic, 7 out of 11 patients had an increase of at least 10% in their BMI after taking the antibiotic. This is quite a change! The authors hypothesize that the antibiotic use led to increased colonization of lactobacillus, which ultimately led to the weight gain.