Hmm...I haven't heard of B6 being an issue, but a great question to ask your OB. I did unison/B6 and it worked very well. I found that you do have to take it regularly/consistently to keep MS away and it takes a few doses to start working.
Here, found this on Kellymom:
Water soluble vitamin supplements (e.g., B vitamins, vitamin C) taken by the mother usually increase breastmilk levels. Breastmilk levels of some water soluble vitamins, such as vitamin C, only increase up to a certain point, then remain steady – even if mom increases her dose.
Mom to two amazing DDs ('07 & '09) and a fur baby.
Gluten free since Nov '11 after non-celiac gluten sensitive diagnosis. Have had great improvement or total elimination of: migraines, bloating/distention, heartburn, cystic acne, canker sores, bleeding gums, eczema on elbows, dry skin and scalp, muscle cramps, PMS, hair loss, heart palpitations, fatigue. I'm amazed.