I don;t need a piece of paper to tell me if I'm health enought to control the aircraft in a safe mannor.
You see, that's the actual problem here that the FAA has the real problem with. It's not the drug or the attitude or any of that other stuff that is commonly considered the problem. It's what the drug doesn't do when absent that's the problem.
The actual problem is this: The part of the software (brain) that determines if it's not operating properly is not operating properly and must be intentionally altered to allow it to know when it's having a problem. IOW, without the drug, the individual is unable to determine if they are screwed up and/or not dealing with reality correctly. Since the defective master caution panel in the brain says everything is ok, the person continues to operate like it is ok even when it's extremely blatantly not right in the real world. (Ever followed a failing artificial horizon that doesn't have a gyro failure flag while under the hood? Same thing)
It's not like the flu or some other straighforward physical plague like that. Icky stomach, puke repeatedly for hours, I'm sick therefore I need to sit at home or go to the doctor until I'm better. Mental problems that are not under control are like; icky stomach, puke repeatedly for hours and while puking your shoes up on other people, you're thinking you're perfectly healthy and the people that are complaining that you're puking on them are obviously crazy. (That's oversimplified however the concept is valid)
Add in that people on medications for mental problems tend to go off their meds because they don't think it's really necessary for whatever reason and they are unable to realize their perception of reality is not necessarily not real. It's a runaway negative reinforcement failure. The longer they're off the meds, the less likely they are to notice they are not functioning properly and need the meds..thus less likely to identify the problem..so they continue to not take the meds. They think everything is perfectly ok with them while in reality everyone else in the world is trying to stay back from the fruitcake that has gone screaming nuts bonkers. At least a drunk can put the beer down and look at their wrecked car the next day and realize they screwed up and shouldn't drink and drive. It doesn't work that way when the drug is used to keep you from crashing your car in the first place.
The FAA's job via medicals is not to be an ass or ground anyone. They are actually there for the better good of everyone across the board even if they are a flaming PITA sometimes. It's to make sure a pilot will not go bonkers and kill a bunch of innocent bystanders or themselves two hours later or a year from now. From their observations, lithium is typically used to control the problem of not being able to know there's a problem which is a big warning sign to them. It's then up to the individual and that individuals actually qualified doctors (not unqualified GP types) to prove that the lithium is not essential for the individuals ability to determine real reality even when they go off the drug for weeks at a time. Fail the reality check without the drug and the FAA will justifiably ground you.
And don't poop on Doc Bruce unless you are willing to take him on in front of the entire FAA medical board. He specializes in the hard to certify cases and he's been doing it for well over a decade that I know of. There are lost causes flying today because he walked them through the process and those individuals did what he asked them to do. Some succeeded, some didn't, it just depended on the situation however there are a lot of people flying today that wouldn't be if it wasn't for his help. If anyone can get you in the air on Li, he can, and if he can't, which he probably can't, you're most likely stuck on the ground if you're going to play by the established rules. If you want to change the rules, line up your ducks and gate crash OKC with repeatedly verifiable proof that they are wrong and you are right. Doc Bruce will do what he can to help anyone however he will only be useful to you if you are on his side of the battle and not shooting turds at him. If you don't like what he says or asks you to do, that's your problem, not his.
