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I recently went to Drs as constipated and getting strange wave like feelings across stomach (lets not go into too much detail here). Prescribed lactulose so def OK in PG. Obviously if you havent got any get it free on prescription!
Yes, lactulose is fine and you can also use Fybogel. I take it you already have some? I know you can get free scrips if you're pg but if you want to bypass the docs you can just buy it OTC from pharmacy.
We all used to swig it on the ante-natal ward and it's fine throughout pregnancy. It didn't work its magic very well with me, though. I ended up on prune juice!! You could try that if you want. I think lactulose tastes VILE