Such acknowledgment for using drugs should only be followed in our personal cases as serves as a line of defense - with all  consequences of this. And I unerstand your post in such context.

But not when we are witnesses. In general, admitting do use drugs substanties a reason for calling us a witness in particular case and the case itself. It also expose ourselves to unnecessary additional questions and focus on us - what we should avoid. 

So if we considered prons and cons of coming out as drug user in our case - that's an option 
In not our case - would stongly reccomend to not go this way and do not acknowledge anything. Just follow 'i dont remember, I dont know" path.