Allergen

An allergen was any substance that could cause an allergic reaction.

In 2377, The Doctor gave Neelix something to counteract any allergens he might encounter in the atmosphere of a planet he was about to visit. 

Known allergens

 * Bee venom
 * Benzethonium chloride
 * Chocolate
 * Icoberry juice
 * Inkarian wool
 * Insect bites
 * Leola ointment
 * Metorapan
 * Retinax V
 * Water
 * Viscoelastic polyurethane foam