ALWAYS READ THE LABEL 
One morning, getting ready to go out for a walk, I was asked to buy some antihistamines. I knew what type was needed, so I checked: “is that the Benadryl, in the yellow box?” Since there are so many types – cetirizine, loratadine, acrivastine and so on – once someone has finally come across the one that finally works, you must follow their instructions very closely.
In my nearest Asda – if you’re American, read Walmart, because Asda is owned by them - there was only one box of the correct medicine on the shelves: Benadryl’s One-a-Day cetirizine, and it was in a box of thirty tablets for £9.50. I seriously considered walking to the next store to try there, but I then saw Asda’s own-brand cetirizine, again a box of thirty, with exactly the same dose per tablet, for £2.50.
The story should have ended there, but I did think that, if I explained why this different box would be just as good as the branded product, it will make sense. However, I compared the descriptions: the full name of the medicine in Asda’s box was cetirizine hydrochloride – C21H25ClN2O3, a combination of carbon, hydrogen, chlorine, nitrogen and oxygen atoms – while Benadryl One-a-Day was cetirizine dihydrochloride, with an extra chlorine and hydrogen atom attached to it.
I made a snap decision, probably one that Johnson & Johnson, owners of Benadryl, wanted me to make – there was slightly more to the Benadryl than the supermarket’s own-brand product, and that, plus I didn’t have to walk any further than I originally planned. I bought wat I was asked to buy, and made for home.
Naturally, I still wanted to know how effective cetirizine dihydrochloride is in comparison to the other kind, so I looked it up and... none has been detected. It is quite possible that Johnson & Johnson made a very slight change to the existing molecular structure of cetirizine in order to register a patent, but I have no evidence to confirm that a common practice in medicine happened in this particular case.
Quite obviously, the power of advertising worked. Fruit and vegetables are the same from one place to the next, but the more particular you can make something, the more exclusive and individual it can be, and the more essential you can make it, the more you can charge - being cynical is a two-way street as far as advertising goes, because it still works no matter how educated you are of its effects, and how dismissive of it you are as a result.
Arriving home, I handed over the Benadryl, saying not to worry about the extra cost for the tablets, as I had been given material for an article – I wait to see if, next time, I will be asked about buying the antihistamines in the yellow box.