KO drops are phenobarbital solution which will be mixed in soft or alcoholic drinks to sedate the victims in the red light areas in Western countries.
They are robbed afterwards. The long time acting barbiturate, Phenobarbital makes you to sleep well long. This is a very big problem in the red light areas in the Western countries and many lose their money and their personal documents. The police warn the tourists to be careful in such places.

Never open your purse and never show the whole currencies you have in the red light area and keep currencies in small lot separately in your pocket or bag.

Other sleeping pills and sedative drugs could also be used to this purpose.

Surely you have also read about the train victims in India. This idea is also the same as KO drops. Do not accept any drinks from unknown persons who are very friendly.