There are six different kinds of herbs.
Three are very common: Brown Twyre, Black Twyre, and Blood Twyre.
The three common herbs share a distinctive "fly buzzing" sound.
Brown and Blood Twyre resemble tall stalks of wheat, with brightly colored grains. Black Twyre is dark, leafy, and grows low to the ground, so it can be easy to miss.
There are also three rare herbs: Swevery, Ashen Swish, and White Whip.
Ashen Swish makes a light rattling sound, while Swevery and White Whip make a rattling and whispering sound.
Swevery only grows outside of the Town, to the South. Ashen Swish is found the most in the Cemetery and the Cape.
White Whip is only found in the Town. Worms sometimes carry White Whip for 7 Bartering after the Haruspex has healed the schism within the Kin in the Abbatoir. Roots (especially ones in infected districts) will occasionally offer it. On Day 8, the island at the foot of the Polyhedron becomes accessible, and several White Whip plants can be found there.
All herbs can also be found at night by the glowing fireflies they emit.
You can start brewing Tinctures as the Haruspex once you fix the smaller alembic in the Lair. Antibiotics and painkillers can be brewed once the larger brewery is fixed.
Tinctures, painkillers, and antibiotics can be used in both the treatment of others and yourself. Painkillers take the place of morphine, and antibiotics take the place of pills like ferromycinium (Medrel), monomycinium (Zürkh), and neomycinium (Yas).
To obtain infected organs without a reputation loss, you can perform autopsies in the Theatre. Besides this, you can find infected corpses or kill Plague victims around town. Some of the organs of an infected person may be healthy.
The better the operation tool, the more likely you are to successfully obtain an organ. The lancet and the Menkhu's Finger are the best operating tools. The heart and the brain are the most difficult to obtain, and the brain does the most damage to the tool you use. It is advisable to go for the liver and heart instead, especially if your tool is close to breaking.
Additional brewing slots can be added to the larger brewery, which is beneficial as the medicine brewed in it can take up to two hours to prepare.
Pathologic Wiki Entry: Townsfolk: Some Townsfolk, like the Herb Brides and Infected Townsfolk, will trade for tinctures. Others will trade you items that you might need for repairing the alembic and brewery.
Pathologic 2 Steam Discussion - Better Diagnostic Tactics with Math! (Archive): A guide on how to best use your tinctures and antibiotics while diagnosing patients. Thanks for the link, Jude!