Data analyzing Zyrtec’s crossover found many patients didn’t follow the drug over the counter, according to a Nielsen-Wolters Kluwer Health report.

According to the findings, only 27% of prescription Zyrtec patients switched to Zyrtec OTC, which launched in January following a December 2007 patent expiry.

That 27% represents roughly 6% of the total number of Zyrtec OTC users, the report found. The majority of Zyrtec OTC users (84%) came to the brand from other OTC products.

“This is the first time anyone has looked at a medication market with Rx and OTC utilizations as a whole rather than as independent components,” said David Martin, vice president of sales and marketing, Wolters Kluwer Health, in a statement. “What we found goes against preconceived notions about a brand switch to OTC. For one, it’s not safe to assume that a switch automatically leads to lower out-of-pocket for the patient.”

Fifty billion dollars worth of prescription drugs are expected to go off patent by 2012, and many will be candidates for OTC, according to Wolters Kluwer Health.