The rarest allergy most often cited in medical and allergy sources is aquagenic urticaria – commonly called a “water allergy.”

Quick Scoop

Aquagenic urticaria is an extremely rare condition where any contact with water on the skin (tap water, sweat, rain, even tears) can trigger hives within minutes.

Because so few cases have ever been reported (rough estimates are on the order of a few dozen worldwide), it’s frequently described as one of the rarest, if not the rarest, allergies known.

What “rarest allergy” really means

There’s no single official global ranking, but specialists and clinic articles consistently put water allergy at the top when they talk about ultra-rare allergies.

Other contenders that are also extremely rare include:

  • Solar urticaria (sun allergy): Hives after sun/UV exposure; only a few hundred documented cases worldwide.
  • Vibratory urticaria: Hives triggered by vibration, like using power tools or riding in certain vehicles.
  • Alpha-gal syndrome (tick‑related meat allergy): Unusual and increasingly recognized, caused by a sugar molecule from certain tick bites, making people react to mammalian meat.

Still, when people ask “what is the rarest allergy,” the answer you’ll most often see from medical and health organizations is aquagenic urticaria – water allergy.

Information gathered from public forums or data available on the internet and portrayed here.