Does biaxin (clarithromycin) cause water retention or weight gain?

No: Really unlikely. Antibiotics work because they hit the germ and not us. Biaxin (clarithromycin) doesn't interact with the kidneys or with fluid. Rare (really really rare) reactions like allergies might possibly cause unexpected effects like liver failure. but Biaxin (clarithromycin) doesn't commonly do this AND, there'd be a lot of other symptoms (brown urine, abdominal swelling and pain, being really sick, etc.).