That Old Time Prescription

While most medications are a mystery to most audiences, there are a few whose native source is more familiar. Willow bark as a precursor to aspirin, or bread mold as that of penicillin, are the usual examples; others, such as poppy extract for a sedative or foxglove extract for heart trouble, are more obscure, but still recognizable to viewers who take an interest.

Therefore, in order to show that a primitive healer, herbalist, or apothecary is genuinely skilled, not ignorant or a fraud, they're shown administering one of these (very) few recognizable proto-drugs to patients in need. Never mind that an historical physician of the era may not know about the medicine in question, or be capable of extracting the active substance from its raw state, or that another, little-known remedy from the period might be safer and more effective: it's always the treatment audiences will recognize that gets used.

Can be an example of Viewers Are Geniuses, for the more obscure remedies. Subversion of Artistic License Pharmacology. Does not apply to fictional medications or recreational stimulants, as use of such drugs does not demonstrate legitimate medical know-how.

Film

 * Disney's Pocahontas has Pocahontas give John Smith willow bark for the pain after he is shot in the side.

Literature
"Bursar: Willow bark. Senior Wrangler: That might work. It's an analgesic. Ridcully: Well, possibly, but he might be better putting it in his mouth."
 * This is done in the Circle of Magic series even to the point of actually using willow bark tea by name for headaches and fevers and the like. At one point there is an epidemic and very modern steps to quarantine the disease and develop treatments through experimentation are undertaken. Somewhat justified in that magic has allowed people in that universe to be much more knowledgeable about the mechanics of the world.
 * In one of the Lord Darcy stories, set in the present day but in an Alternate History where magic has been developed as a science (meaning, among other things, that medical science has not developed because healing magic makes it unnecessary), two wizards have a conversation about unlicensed healers who do things like treat wounds with mouldy bread and heart trouble with "a tea brewed of foxglove"; they regard these as superstitions, since there's nothing in the laws of magic to justify them working.
 * In the Discworld series:
 * The moldy bread poultice is one of Magrat Garlick's specialties. In Carpe Jugulum, Magrat also makes willow bark tablets.
 * It also gets mentioned in Hogfather:


 * Brother Cadfael is all over this. He was a soldier in the Crusades, and he's an apothecary in his monastic community. Lavender for headaches (The Price of Light), poppy extract, and so on.
 * In the Earth's Children series it's mentioned that practically everybody knows about willow bark tea, even non-healers.
 * In the Garrett P.I. fantasy/noir series, Dean brews willow-bark tea for Garrett after nights of heavy drinking.
 * In the novel Outlander, Claire Beauchamp demonstrates even more knowledge when she comments that willow bark tea can make bleeding take longer to stop while discussing the healing properties of herbs with the keeper of Castle Leoch's herb garden.
 * In A Song of Ice and Fire, maesters (essentially doctors, though they have other duties) commonly prescribe "milk of the poppy" (that is, opium) to anyone suffering from a particularly painful injury.
 * In Lois McMaster Bujold's The Hallowed Hunt (part of the Chalion series), Ingrey was given a pain-reliving medication made from poppies (among other things).
 * Bread mold is among the assorted herbs kept in Vlad Taltos's witchcraft supply pouch, presumably for use on wounds. In Athyra, some of the treatments mentioned by Master Wag are also this trope.
 * Willow bark tea shows up in the first Arrows novel. There's also a reference in the Collegium Chronicles to Bear's attempts to preserve bread mold for wound treatment, instead of shipping moldy bread out from Healer's Collegium and hoping the wrong type of mold doesn't develop.

Live Action Television

 * There is an episode of Dinosaurs in which the baby gets seriously ill and the family spends lots of money on fancy new medicines. When those fail, they go to a healer who lives in the woods, who cures the baby with moldy bread.
 * In one episode of Queen of Swords almost everybody in the village gets sick; the Mighty Whitey doctor doses them with a practically magical healing elixir, which he later reveals is made from willow bark.
 * Dr. Quinn Medicine Woman was always prescribing willow bark tea.
 * She is also seen prescribing digitalis to a woman for a heart murmur. She also prescribes several things given to her by the Native Americans, much to the town's distrust and horror.