I understand Rich theory on this but I can say that, from my own experience, that taking b2 with no b complex (I still take about 100 mcg of b12 daily, with magnesium and manganese) feels totally different than when I take a b complex. I didn't feel anything from a b complex while b2 alone quickly reduced some symptoms I had for years, like purple nail beds (cyanosis) and that says a lot! I also have a strong metal taste in mouth at times, which is expected when excess iron is released from liver according to the theory. I don't know if I'll end up with other B deficiencies that limit my progress at some point, but clearly people don't have a startup effect from the same amount of b2 in a B complex. Don't know about b12 specifically though. 10 mg of B2 alone was enough to cause a big startup. Hope this helps!