In my part of the country, it's seriously allergy time. My car is covered in a green haze, and my eyes and nose are running like they're in the Penn relays. The ads promise me "Claritin clear" days if only I would take 10 mg of loratadine. One ad features an allergy sufferer who takes only the brand-name version (Claritin) to be sure that he gets exactly what his doctor used to prescribe. Viewers are cautioned that some other allergy medications might make you drowsy and shows a box with the word diphenhydramine circled in red. It all vaguely implies that if you don't take Claritin itself, you might get something that will put you to sleep. If you know your chemistry, you might realize that diphenhydramine is actually the generic name for Benadryl (which can in fact make you drowsy). If you take Benadryl, generic or otherwise, you could drift off to sleep in the middle of that critical meeting. But if Claritin doesn't make you sleepy, neither will any generic brand of it. Bottom line: if it says loratadine on the box - it's the same stuff your doctor used to prescribe as Claritin. No need to buy the brand name box.
Both loratadine and diphenhydramine are H1 blockers. They bind to a particular type of histamine receptor, and sit there. An overabundance of histamine, released in response to an allergen leads to the woes of pollen season. If the histamine can't get into the receptors, it can't make you miserable. Interestingly, histamine production also plays a role in acid reflux. Several good heartburn drugs (Zantac and Tagamet) are H2 blockers.