The warning on the back of the package of Halls Defense cough drops I just purchased says the following:
[...] take one drop 4 times per day. WARNINGS: Do not use for more than 8 weeks consecutively.
I’m guessing the warning is because, after 8 weeks of four of these days a day, you probably have type-2 diabetes. The first two ingredients in these things are sugar and glucose syrup. Aside from that, if you’ve had a cold for 8 weeks and are still seeking a solution in cough drops, you have other more pressing health concerns.