P21 (P021)

Neurogenic peptide derived from the CNTF active region — promotes new neuron growth and reduces tau pathology.

P21 is an experimental compound built from the active piece of CNTF (a nerve growth factor) and engineered to actually cross into the brain. In animal studies it does two striking things: it stimulates the birth of new neurons in the hippocampus (the memory centre) and reduces the tau tangles tied to Alzheimer's. That makes it a real neurogenesis-and-neuroprotection candidate rather than just a focus aid — but human data is essentially nonexistent and it's expensive, so it sits firmly in the experimental tier.