“They found increases in the number of dendritic spines and in their size within 24 hours of administration of psilocybin.”
One dose gave mice about 10% more connections between neurons, and they were still there a month later. Chronic stress and depression strip those same connections away. The study is in mice, so hold the headlines. But a human trial already had psilocybin matching a standard antidepressant, and the drug is still illegal almost everywhere.