B-12 Vitamin safe?

Is B 12 (the kind that absorbs under your tongue) safe to take while TTC? And is it safe to take while also taking prenatals?? My husband and I are both fairly active and we eat healthy but we have terribly slow metabolisms. My mom suggested B12 because it's helped her and my sister lose weight by correcting their metabolism... but I'm worried that if I turn up pregnant without knowing it, due to my irregular cycles, it may be unsafe... Also I don't want to overdo anything while taking two vitamins at once (Prenatals/B12)... I've tried researching it and there's just so much contradicting information I don't know what to trust...