Suing for medical malpractice requires time and deep pockets. A medical practitioner can never admit wrong if they wish to be covered by their malpractice insurance.
What you are describing isn't uncommon. Some doctors will go off-protocol because they are following a study that is looking like it has better results. Other times, they don't agree with the treatment at all.
We have had a similar case in Australia recently involving chemo under-dosing.
If this example is anything to go by, the under-dosing was systematic and affected many people. Further, it was found that a number of doctors in this circle had engaged in similar practices.
If you are part of a community where you are able to track down other patients of this doctor, you may find one or more of them with deep pockets or may have the numbers to look at a class action.
I have been involved with a number of medical crowd-funding campaigns and also a number of legal crowd-funding campaigns and I do feel that crowd-funding legal fees for medical malpractice will be quite difficult as your audience is small. People are much more likely to donate to save a life, or to defend against an injustice. Having said that, crowd-funding exists for exactly this reason and I do believe that you should attempt a campaign on multiple platforms.
I would look at some of the more well known platforms plus some of the specialty platforms. I would also Google search for any successful campaigns for lawsuits and/or medical malpractice.
I would suggest GoFundMe, Funded Justice, and CrowdDefend for a start.
If you haven't done a crowd-funding campaign before, I would suggest engaging someone who has. Many people skilled in campaigns may do it for a share, an upfront fee, or publicity. The amount of funds you will need to raise will be unlikely without wider media and community involvement. Get in touch with advocacy groups.
Before starting the campaign, I would suggest a legal consultation and also a 2nd or 3rd medical opinion. I would even ask the doctor if they would be willing to testify in court on standard dosages (and range of doses). I would document any reasoning from your doctor WHY they are giving you these doses, if they are standard, if this dose is given to all patients or a personalized dose based on other medical factors. I would also make a list of anything you may have conveyed or any behavior that could be used in a defense. This includes not following guidelines on diet or drug/medication use. I would make sure that the alternative doctor(s) and legal counsel are well aware of these.
The path of least resistance is to simply change doctors. The two motivators for a lawsuit are: a) Justice through doctor's possible loss of license, income, or wealth; and b) a pay day. If you were not permanently harmed, winning a large amount of damages (large enough to cover legal and the years in court) will be unlikely without the best legal representation.
Our medical, legal, and crowd-funding systems all have room for improvement.