Wormwood

Let's take a look at this snippet from Owlsight by Mercedes Lackey. Healer Keisha is being interrupted in the middle of decanting an herbal potion:

"Keisha!"
Keisha Alder ignored her sister Shandi's continued calls; she was in the middle of a job she had no intention of cutting short. The sharp smell of vinegar filled Keisha's workshop, but she was so inured to it that it hardly even stung her nose. Shandi could wait long enough for Keisha to finish decanting her bruise potion, staining out the bits of wormwood with a fine net of cheesecloth. Keisha wrinkled her nose a little as the smell of vinegar intensified; the books said to use wine for the potion, but she had found that vinegar worked just as well, and there was no mistaking it for something drinkable - unless your taste in wine was really wretched. A cloth steeped in this dark-brown liquid and bandaged against a bruise eased the pain and made the bruise itself heal much more quickly than it would on its own, so despite the odor the potion was much in demand.

Wormwood (Artemisia absinthium) is better known for its healing properties where digestive disorders are concerned, but it has traditionally been used in poultices and salves to treat bruises and insect bites. It contains tanning and bittering agents, which have astringent effect and stimulate the blood flow, which could account for its healing properties where contusions are concerned.

A medicinal plant's healing agents are usually extracted via alcoholic or oleaginous solutions, but vinegar also serves to extract most alcohol soluble ingredients, so Lackey's paragraph above is correct in that aspect, as well. The reference to using vinegar so the potion would be undrinkable probably refers to the fact that wormwood contains thujone, which can be toxic in high dosages. It may induce miscarriage and harm a nursing infant.

The once-popular drink absinthe contained a high percentage of wormwood, and was said to cause hallucinations, resulting in the spirit being banned in many countries in the early nineteen hundreds. The ban has since been rescinded, though only if the thujone is below a certain threshold.