Would you avoid capecitabine in favor of fluorouracil in the adjuvant treatment of colon cancer for a patient with history of a gastric bypass?  

Yes, that would be a good reason to use adjuvant infusional 5FU over capecitabine due to concerns about absorption. In general, 6 mo of FOLFOX would be used in lieu of 3-6 months of adjuvant cape/ox. Here is a reasonable resource for this decision, showing faster absorption of capecitabine in gastre...