Doxinate OD is a doxylamine and
vitamin B6 combination, one of the better studied options for nausea in pregnancy, and it's generally considered safe to continue through pregnancy when it's genuinely needed. Nausea coming back each time you stop it doesn't mean something is wrong, it usually just means you still need the medicine.
Next Steps
Go back to your obstetrician and tell them the nausea returned every time you stopped the tablet. They can decide whether to restart it and for how long, based on how you're doing overall.
Health Tips
See your doctor sooner if you're unable to keep food or fluids down, losing weight, or feeling very weak, since that needs more than tablets alone.