My son took a short course of keflex after a ureteral stent being placed for UPJ obstruction, and now when he takes acetameniphen for low level discomfort from the stent, his skin tone becomes bluish. Cyanosis appears to be one of the potential side effects from both of those meds, but he s done with the keflex (been two days off). I don t want to bother his surgeon with this if it s not a big deal, and don t want to tell him he can t have relief from the periodic discomfort. He has no other apparent adverse reactions from the meds going on - just this blue/green pallor when he takes the acetominophen.