Can zofran (ondansetron) cause chronic constipation that causes chronic nausea and ultimately weight loss?

Ondonsetron: Zofran (ondansetron) treats nausea and vomiting from cancer chemotherapy and to prevent and treat N&V after surgery. Side effects include diarrhea, headache, fever, lightheadedness, dizziness, drowsiness, constipation, rash, blurred vision and muscle spasm, but whether your symptoms are due to this only your treating doctor can say. Weight loss should not occur unless your intake is markedly less.

Anything is possible: But it's not likely that Zofran (ondansetron) would cause enough constipation to cause enough nausea to cause weight loss. In fact, Zofran is used to treat nausea. Check out http://www.drugs.com/sfx/zofran-side-effects.html where I couldn't find any mention of your scenario. If you're worried about that sequence of events, ask your doc for another antinausea medication.