Hydrocodone allergies--how do they happen?

Peripheral effects. There are several mechanisms, one a true ige allergy to hydrocodone causing an immediate anapylactoid and or anyphlactic response due to mast cell mediation. More commonly the conversion of the hydrocodone to norhydrocodone or hydromorphone causing histamine release by binding to peripheral opioid receptors can cause an allergic like symptomatology or pruritic reaction.