Dihexa is reported to be seven orders of magnitude more potent than BDNF itself at promoting synaptogenesis in cell culture. It also has essentially no human data. That tension — extraordinary in vitro potency, near-zero clinical evidence — defines everything you need to know about where to place it.
Cerebrolysin is the only compound in this review series with multiple Cochrane reviews and regulatory approval in Europe and Asia. It is a peptide mixture, not a defined molecule — which makes it harder to characterise and harder to source reliably.
Semax is a synthetic ACTH analog with two decades of clinical use in Russia for stroke and cognitive disorders. It has more human evidence than almost any other nootropic peptide — and most Western researchers have never heard of it.