Coca cola.

Coke was initially sold as a medication.

In the 80's, blind trials showed that people preferred the taste of Pepsi - but when they knew the brand, they preferred Coke. Coca cola executives were so worried by this that they launched "New Coke" which was more like Pepsi, and people preferred it in blind tests. But it failed utterly, and the original Coke itself continued to be popular, as it has been to this day.

See the "Rx for Success" side bar on page 2. The last two items are branding and clever names.