Rosemary
The Essentials

One of the best-studied culinary herbs for cognition — aroma and low-dose extract trials show small but reproducible working-memory and mood effects.
Persian medicine's classic warm & dry cephalic — the herb for a dull head, weak memory, and cold, slow circulation.
Standardised extracts (~500 mg dried leaf / day) show modest anxiolytic (calms anxiety) and glycaemic effects in short RCTs.
Topical (put on the skin) rosemary essential oil (the concentrated aromatic oil of the plant) at ~2% matched 2% minoxidil for androgenetic hair-count over six months in one head-to-head trial.
Culinary rosemary is deeply safe; concentrated essential-oil doses are the safety line — never taken internally, avoided in pregnancy and in poorly controlled epilepsy or hypertension (high blood pressure).