Testing of Claritin showed that it's ineffective for most users unless 4 times the dose is used. At that dose it tends to induce drowsiness.
https://www.nytimes.com/2001/03/11/magazine/the-claritin-effect-prescription-for-profit.html
The effectiveness of loratadine had been an area of F.D.A. concern as early as January 1987, just months after Schering filed its application. Dr. Sherwin D. Straus, the F.D.A. medical officer assigned to review the drug, told the company that a 10-milligram dose of Claritin -- the amount marketed today -- did not appear to be very effective. He reiterated that point in public on Oct. 23, 1987, when the F.D.A.'s Pulmonary-Allergy Drugs Advisory Committee met at the agency headquarters in Rockville, Md., to consider Schering's application for loratadine. (This panel of outside experts doesn't formally approve new drugs, but makes influential recommendations to the agency.)
I've always had better results with 60 mg Allegra taken during the day. With the generic a pharmacist said it was OK to split a 180 tablet to use twice a day. At night nothing works better than diphenhydramine. If my allergies are really bad I'd double up with both. A pharmacist told me it was OK.