Hi: Why Is The Prescription Alegra-D (24Hr.) Different From The

Hi: Why is the prescription Alegra-D (24Hr.) different from the over the counter Alegra-D. Pills are different. I live in Oregon and can not purchase over the counter, have to go into Idaho to get it. The Dr. wrote a prescription and I got it filled in Oregon. Completely different pills.
What is a safe amount of time to take these pills.?
Thanks for your assistance in these questions.
Patricia