For instance, Weed might scramble up your memory. Basically either fogging over it or, putting some strange ones in place. Random giggling might also be a good thing. Based on the strength of the weed.
While Lana might scramble numbers you input into a soup. Making entering codes a problem. phone numbers a challenge, etc.
While Marcy might cause you to do mundane things quicker and twice. Basically emulating tweaking. You might disassemble your gun and then reassemble it and disassemble it again. Shower, get outta the shower and automatically activate the dryer, but go back for a second shower etc.
Things like that, basically innocuous effects to just slightly or majorly inconvenience drug users. But also reinforce that you aren't in full control of your actions.
Seriously any other things you can think of that might be good suggestions, I've barely experienced drugs on any of my character. And so I don't have the experience on what is supposed to do what, it just seems weird that withdrawals are basically the only place where they remove your ability to function normally at times.