An allergy or allergic reaction referred to a body's inappropriate immune response to various stimuli, usually proteins.
The body's responses could be minimal, but could also include anaphylactic shock or even death. Allergies often had a genetic basis. They could be present from birth or could present themselves at any later point during a lifetime.
- The inhabitants to Sirius IX were allergic to a love potion sold to them by Harry Mudd. (TAS: "Mudd's Passion")
- Humans were allergic to Ferengi blood pyrocytes. (TNG: "The Price")
- Joined Trill were extremely allergic to insect bites. The reaction caused by the insect's venom interfered with the biochemical connections between the host and symbiont. (DS9: "The Siege")
- Henry Janeway was allergic to chocolate, preferring oatmeal cookies instead. (VOY: "11:59")
- Travis Mayweather was allergic to a pain medicine he was once given on Risa. (ENT: "Two Days and Two Nights")
- Malcolm Reed was allergic to bromelain, for which he took regular injections. He was also allergic to dust mites, oak pollen, and tropical grasses. (ENT: "Silent Enemy")
- James T. Kirk was allergic to Retinax V, an eye medication. (Star Trek II: The Wrath of Khan)
- James T. Kirk was allergic to the vaccine for the Melvaran mud flea virus. (Star Trek)
- Wesley Crusher was allergic to metorapan, an analgesic medication. He was treated with bicaridine instead. (TNG: "The First Duty")
- Quark once stated he was "allergic to nature." (DS9: "The Jem'Hadar")
- Commander Zakarian was allergic to many things earning him the nickname "Old Sneezy". (VOY: "Caretaker")
- Jadzia Dax was allergic to icoberry juice – it made her spots itch – but she drank it anyway because she liked the taste. (DS9: "Let He Who Is Without Sin...")
- Tom Paris had an allergic reaction to the water Neelix used for the coffee "Paris Delight". (VOY: "Threshold")