iTZDodge answered it pretty well, but here you go:
The conversion of L-DOPA to dopamine is heavily dependent on an enzyme known as DOPA decarboxylase (DDC). Of these decarboxylase enzymes (there are several different variations), the "best" one for this conversion process is called DOPA decarboxylase. Although DOPA decarboxylase can be indirectly affected by several different nutrients (specifically shortages of nutrients), the main one involved in this step is called pyridoxal phosphate. Pyridoxal phosphate is the chemically "active" form of vitamin B6. Generally the greater the amount of B6 present, the higher the level of DDC.
High DDC levels tend to be disadvantageous to HGH-Up users, as L-Dopa can cross the blood brain barrier relatively intact- but dopamine cannot. L-dopa must stay intact to elicit increases in GH, and if it doesn't, the net effect will be a decrease in product efficacy. If there is too much DDC present because of high B6 levels (which occur from doses of 10-15 mg of vitamin B6), there will be a very large increase in the conversion of L-Dopa to dopamine peripherally, and because dopamine cannot cross the BBB, the target effect of increased GH most likely will not occur, at least optimally.
There is a silver lining- because it is water-soluble, B6 has a very short half-life (around 4 hours), so levels of B6 will not remain elevated for extended periods of time, so if the dose of B-6 is split away from your dose of HGH-Up by a few hours, it should be ok, at least from what I have seen......