Try taking them with food even if it's a handful of crackers and a smoothie. If you can eat regular meals then take it within 20 minutes after eating. Also if it has synthetic vitamin b in it, it could be hard on your body. Try a gel based prenatal pill with cyanocobalamin for the vit b 12, the methylcobalamin is synthetic and hard for some people to digest.
Have you tried taking them with food? I had to do that in the beginning because they hurt my stomach so bad!