The present invention provides for a storage stable pharmaceutical liquid solution for oral administration having a pharmaceutically effective amount of an antihistamine and having a purity equal to or greater than about 99% by weight-based HPLC assay, residual solvents of less than about 0.5%, and a total impurity of less than about 0.2%. The storage stable solution preferably contains cetirizine. The present invention further provides a process of preparing the storage stable pharmaceutical liquid solution as well as a method of treating a mammal with a therapeutically effective amount of cetirizine in the stable pharmaceutical liquid solution.
