Monoterpenes

Any plant with a distinct smell is worth suspecting. Monoterpenes are concentrated in glandular trichomes, resin ducts and oil cells, and a few families are famous for them:

Because they are volatile, content varies sharply with harvest time, drying and storage — an old, badly stored aromatic herb has lost much of what made it medicinal.

Two isoprene units join to give a ten-carbon skeleton, which then either stays open-chain or folds into one or two rings. Oxygen is added afterwards, and the oxygen-bearing form usually carries the pharmacology:

Hydrocarbons

limonene, the pinenes, myrcene

Alcohols

menthol, linalool, geraniol, borneol

Aldehydes

citral, citronellal

Ketones

camphor, thujone, pulegone, carvone

Oxides and esters

1,8-cineole, linalyl acetate, bornyl acetate

That ladder matters for safety as much as for scent: the alcohols are generally the gentlest of the group and the ketones the most hazardous.

Monoterpenes are lipophilic and small, so they cross membranes easily — including the blood-brain barrier, which is why inhaled aromatics can act on mood and alertness within minutes. Several are direct receptor ligands: menthol activates the cold-sensing TRPM8 channel, and linalool and 1,8-cineole have documented anxiolytic and airway effects.

Clinically the group underpins the carminatives (relaxing gut smooth muscle), the expectorants and mucolytics of the respiratory herbs, the topical rubefacients and counter-irritants, and a large share of plant antimicrobial activity — thymol and carvacrol are genuinely potent against bacteria in vitro.