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

External link

Materials
Medicine