Fexofenadine hydrochloride,
the active ingredient of ALLEGRA®, is a histamine
H₁-receptor antagonist with the chemical name
(±)-4-[1-hydroxy-4-[4-(hydroxydiphenylmethyl)
-1-piperidinyl]-butyl]- alpha,alpha-dimethyl
benzeneacetic acid hydrochloride. 

It has the following chemical structure:

 

The molecular weight is 538.13 and the empirical formula is: 

C ₃₂ H ₃₉ NO ₄ • HCl . 

Fexofenadine hydrochloride is a white to off-white crystalline powder. It is freely soluble in methanol and ethanol, slightly soluble in chloroform and water, and insoluble in hexane. Fexofenadine hydrochloride is a racemate and exists as a zwitterion in aqueous media at physiological pH. 

ALLEGRA is formulated as capsules for oral administration. Each capsule contains 60 mg fexofenadine hydrochloride and the following excipients: croscarmellose sodium, gelatin, lactose, microcrystalline cellulose, and pregelatinized starch. The printed capsule shell is made from gelatin, iron oxide, silicon dioxide, sodium lauryl sulfate, titanium dioxide, and other ingredients.