- Jaksch's anemia or disease
- Jaksch's anemia or disease's test
- jalap, jalapa
- Jalapin
- Jambul
- Jamestown Weed
- Janet's disease
- Janet's disease's test
- Janeway's pill
- Janeway's sphygmomanometer
- janiceps
- janiceps asymmetrus
- Jansen's operation
- Janus green
- japaconitine
- japan wax
- Japanese gelatin, gelatine
- Japanese liver fluke (flook)
- Japanese river fever
- Japonic acid