Beyond a century sell N-Acetyl L-Tyrosine, supposedly over twice as rapidly absorbed & bioavailable than regular L-Tyrosine. Twice as expensive too, 50 grams, $11.50, but with smaller doses it's actually more cost-effective and certainly easier to encapsulate. I'm probably using 1/4 as much, with the same apparent benefits for concentration and mood.
I'm told the active co-enzyme form of Vitamin B6, pyridoxal-5-phosphate (P-5-P,) is a great addition to L-Tyrosine and potentiates neurotransmitter production.
I've also heard the cheap inactive form of Vitamin B6 Pyridoxine HCL is worthless but can cause a harmless numbness and tingling in the extremities.
Guess which one Biotest and EAS use in their formulas.