The invention pertains to a method for preparing dexchlorpheniramine ("dexchlor") tannate by reacting dexchlorpheniramine free base at a temperature of about 75 to about 150 C. with tannic acid neat or as an aqueous slurry containing up to about 20 wt. % water. The dexchlorpheniramine free base may be obtained by reacting a commercially available dexchlorpheniramine salt such as dexchlorpheniramine maleate with a base such as aqueous sodium hydroxide. The resultant dexchlor tannate has extended release properties and is useful in pharmaceutical compositions as an antihistamine for human beings.