Nasal is the real route for Selank, not just the convenient one. The Russian product was developed and registered as nasal drops. That is the formulation the clinical work used.A mouse comparison gave the same 300 mcg/kg dose intranasally and by injection for five days. Both routes cut anxiety and improved the maze test, but only in the high-anxiety strain, and they did not hit the same receptors. Injection raised GABA binding in the frontal cortex. Nasal raised NMDA binding in the hippocampus and left GABA alone. So the route changes the effect, not just the speed.
Nasal gets a fraction of the dose to the brain through the olfactory and trigeminal pathways within a couple of minutes, with less of it wandering through the blood first. Injection is more precise on the syringe and gives more systemic exposure, which matters if you care about the immune side of Selank. It is the worse match for the anxiolytic and cognitive use, and it has almost no human data behind it.
For what Selank is actually for, use the spray. Typical researched range is about 400 mcg–1000 mcg a day and usually dosed twice. Injection is a gray-market adaptation, not an upgrade.