No, it's not what you think. No really. I happen to take a lot of medications (you know, the ones prescribed by a doctor...). Therefore, I am a little concerned about whether those drugs might have interactions with each other, or perhaps with drugs that I might take in the future. Enter Drugs.com! Everything you ever wished you knew about the drugs you are taking! For example, I am taking an iron supplement and loratadine (Claritin). While food can increase the absorption of loratadine by almost 40%, food can inhibit the bio-availability of ferrous sulfate, which is best taken on an empty stomach. However, iron taken on an empty stomach causes great discomfort, and is therefore often bearable only when taken with food. I have had the pleasure of experiencing this on many an occasion, including almost ruminating on the freeway en route to my sister's wedding. In any case, there were no interactions with the drugs that I am taking. Which is good.
I was also interested in the mechanism of action of loratadine and cetirizine (Zyrtec). Claritin has worked perfectly for me in the past, and still seems to be pretty effective. However, Zyrtec has never seemed to work at all. Well, thanks to Drugs.com, I might have a better understanding.
Both loratadine and cetirizine are histamine antagonists, which do so by competing with histamine for binding to the H1 receptor found on smooth muscle, endothelium, and CNS tissue. Based on the structure of histamine, I can see two possible general binding locations on loratadine and cetirizine, shown in the red and green respectively. While neither of them resem ble histamine exactly, if I had to make a guess, I would say that loratadine has more cognate structures to histamine than cetirizine does. Of course, I am neither an organic chemist nor a pharmacologist. It could, however, be possible that I have a mutation in my H1 receptor gene which might reduce cetirizine's binding affinity. Maybe I'll find out someday, when I have my genome sequenced...