P21 is a small peptide developed in academic neuroscience research, designed to encourage neurogenesis, the birth of new neurons, and to counter some of the processes associated with Alzheimer's disease in animal models. The preclinical work is legitimate research published in peer-reviewed journals. What does not exist is any human data whatsoever. There is no corporate development programme, no registered clinical trial, and no published study in which a person has received it. Everything claimed for it in consumer contexts is an extrapolation from rodent brains, and the history of Alzheimer's drug development is a long catalogue of compounds that looked excellent in mice and did nothing in people.