l-carnitine and acetyl-l-carnitine together Designs for Health Carnitine Synergy - 400mg (from Carnitine Tartrate) + 100mg Acetyl L-Carnitine Pills Acetyl-L-Carnitine (500 mg)
Description
doi: 10.3233/JAD-190074 152 SimpsonD

IV (or intramuscular) delivery bypasses the failing step entirely the classic example being B12 in patients who cannot absorb it from the gut

Those that used L-Tyrosine demonstrated superior performance in the 2-back test , but not the 1-back test

A summary of evidence
