Drugstores and Apothecaries back then were, believe it or not, pretty scientific places. They had scales sensetive enough to measure grains, minims (1/480th of a fl.oz) and drams. All of the poisons that I have that are hand written have big warning stickers all over them. Opium is one of those meds that could croak you very easily. Any druggist would have put CAUTION or POISON on the bottle. It's one of the things that make them so desirable. Also, there would have been dosage indications e.g. "8 drops, 3x/day" or whatever. Anyone using opium would know exactly what was in the bottle and would not have taken time to label the contents. Just my opinion.
Paul