TBH N2L I never found out. At the time I had DD1 on my shoulder with a temp of 40 and I was going to buy the berocca with her antibiotics. Iz was so sick I just wanted to get her home so didn't bother going into it with the pharmacist. Then went onto the berocca website and did some googling and found more "don't do its" but no reasons why. I ended up getting a vitamin B shot from my GP after Christmas but I didn't notice any real energy boost. I put it down to maybe getting too high a dose of something if you were taking them both. I've since discussed my total lack of energy with my Ob and basically reached the conclusion that I'll feel better in another 3.5 months and in the meantime just have to deal with it.