Well, it's certainly healthier for a smoker to switch to vaping, but
not healthier for a nonsmoker to take up vaping, regardless of the nicotine content. Period. There are just too many factors.
Your doctor has a point
to a certain degree. First of all, just about every doctor is leery of anything relatively new unless there has been significant research on it and it has been approved by all the appropriate agencies. Secondly, doctors tend to be doubtful of anything they are not personally familiar with, and nearly all know next to nothing about anything beyond their specialty-- which is one thing that makes practicing medicine more of an art and less of a science. Third, practicing doctors are not pharmacists, most know very little about drugs
comparatively, they mostly know what they are supposed to subscribe for what illness or symptom, some of the contraindications, major side effects and interactions, and that's about it.
I'll compare electronic cigarettes/nicotine juice to nootropics like the racetam family. Both have mild side effect profiles (though racetams are much less toxic than nicotine), both are relatively new to the medical community, both need much more long-term study though what studies do exist seem to reflect an acceptable level of safety with responsible use, both are becoming more popular in the public, and doctors know next to nothing about them. Yet, there is all kinds of pop myth, misleading advertising, political shenanigans, and scary nonsense in the media about them. This the bulk of what the average doctor knows about e-cigs and racetams. So naturally most doctors warn people away from them. In many cases like this, the doctor's opinion can be easily disregarded if you have truly done your homework.
However, your case is slightly different because it involves psychiatric treatment. It is true that regular nicotine use is unlikely to have zero impact on a psychiatric drug regimen, nor is it unlikely to have zero impact on a normal brain, nevermind an abnormal one.
So a schizophrenic under treatment is likely to be on an antipsychotic and probably a mood stabilizer and maybe an antidepressant. Antipsychotics, both typical and atypical, are dopamine antagonists. Their effectiveness in part comes from their ability to reduce excessive dopamine activity in certain regions of the brain. Nicotine does the opposite, increasing dopamine. This is one of the reasons why nicotine has its long-term-use adverse effects profile.
Any drug that triggers an increase in dopamine will eventually downregulate one's dopamine. This is only one way nicotine can cause depression, anxiety, irritability, insomnia, fatigue, and a myriad of other problems. The more you consume and the longer you consume it, the likelier it is to do more harm than good in the end.
Nicotine stimulates glutamate and histamine. Schizophrenic brains classically have abnormal histamine levels (chronically high histamine can cause depression among other things in otherwise normal brains), so this can be a bit of a wild card.
Nicotine and GABA: vaping nicotine causes the brain to produce GABA four times faster than normal. This is part of its quick reward and
initial relaxation, but also part of its addictive potential, and regular occurrence of this will downregulate GABA and make one more susceptible to anxiety, insomnia, etc. I am unsure of GABA's role in schizophrenia but I know it's involved.
Nicotine is cholinergic; most drugs prescribed by psychiatrists are anticholinergic in one way or another, and in varying degrees. Excess acetylcholine causes depression, anxiety and irritability. Depending on what drugs you are taking, nicotine can easily be working against their mechanisms.
Nicotine does many other things I haven't even begun to read about, such as affecting hormones. But generally it seems to have many of the same pros and cons of any other stimulant drug.
A while ago, I did come across a lone study that found a strong correlation with smoking and bipolar and schizophrenia, and hypothesized that there may be something about nicotine that helps symptoms in some way. This study found that when schizophrenic patients ceased smoking, some of their symptoms returned or worsened. I don't have any specifics on it and truthfully am too lazy at the moment to spend 10 mins searching for it on PubMed. I'm sure anyone interested would be able to find it. I don't know the significance of this is because this is a form of self-medication, and schizophrenics also tend to classically self-medicate with alcohol which initially helps, but isn't that awesome for the long run. Alcohol is a GABA agonist.
In short, your doctor is probably right for your individual case. Your own report of depression is a point in the doc's favor. Would likely benefit from significant reduction in nicotine consumption.
I really hope no one gets defensive about what seems to an overwhelmingly negative evaluation of vaping nicotine. This is only one side of the story, and besides, knowing these kinds of things can help people take meaningful action to mitigate adverse effects, such certain alteration in diet, lifestyle including addition of certain supplements, so they can still have their fun with vaping. And in normal brains, with
mild use, nicotine can have some cognitive benefits. If it didn't, I wouldn't be using it.
Disclaimer: I have several docs including a shrink and a therapist. I'm on drugs too. Har har. Even though I have confidence in what I've said and I might even sound like I know what I'm talking about, do not take anything I say at face value--simply because listening to a stranger on an internet forum about things like this is generally bad policy. Look up everything for yourself if you think it is relevant to you.
Safe vaping everybody!