Thickened Formula ?

Has anyone ever had experience with thickening formula with cereal to help baby's spit up? My DD's ped just told us to start this because she is still spitting up a ton even on Zantac. We have tried many different formula brands and feeding options. This is a last resort before the GI doc. Since Monday she has been extra fussy, not eating as well, waking up to eat in the middle of the night, and is still spitting up. Sometimes the spitting up is worse. Any help/tips would be appreciated!!