Can zofran (ondansetron) cause false positive pregnancy test? if so how common is it?

Extremely rare: According to eHealthme, a private consulting company that looked into adverse reactions/side effects of Zofran, only 1 in 22,426 people taking Zofran reported a false positive pregnancy test.This does not mean the odds are 1 in 22,426, but it does shows that it is rarely reported. Phenergan, (promethazine) a different medication that is also used to treat nausea, HAS been reported to cause false positive tests.

Get blood test: If you have a positive result on a correctly done urine, test, it is prudent that you get a blood test for HCG level to confirm or rule out pregnancy. Unless proven otherwise, a positive urine test after a missed period should be considered as diagnostic of pregnancy. For the sake of your and your potential baby's health, you should seek medical care.