Really vague question. Cocaine is a stimulant, so it tends to make people feel more productive, however it has a relatively short duration, so you have to keep redosing if you're trying to use it for that purpose. Personally, I've never found it useful when I'm working or trying to get something done as the short duration and compulsivity of redosing makes me lose focus quickly. If you're not using it while you're working, i.e., the night before, it can certainly keep you up through the night and the lack of sleep can make you sluggish and lazy the following day. I don't know if that necessarily correlates to being incapable of employment, but then I don't really understand what you mean by that anyway.
As far as illegal drugs, you're talking about hundreds of different substances. Some drugs might make you incapable of employment if you get caught or fail a required drug test, but you'll have to be more specific which ones you're talking about. I would say, if anything, benzodiazepines are prone to zombify people and those are perfectly legal if you have a prescription.