I suppose a lot won't do it if they don't feel it has therapeutic value. I know some simply refuse to believe it's of any medical benefit.
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I've talked to my doctor about this in depth. He is very pro MMJ, (actually cannabis in general), but is not pursuing becoming an approved physician at this time.
Physicians are not required to keep up to date on every medical treatment or condition. I don't how license renewal is in other states, but here physicians do have to have CE credits when they renew, but they get to choose the majority of topics/hours. Some are mandated, like child abuse and opioids, but the rest of the CE hours are entirely up to them. Some doctors simply don't research MMJ because of how involved the research into it can be, especially if they want to become approved to certify/recommend patients to the MMJ program. Many doctors are fine with maintaining the status quo, seeing patients for colds, flu, arthritis, etc. and just don't want to get involved in "new" treatments. They have their clientele, so to speak, and are not interested in branching out. Especially older physicians approaching retirement.
Some physicians are waiting for more definitive US research to become available. There's research from other countries but some are waiting to see how the MMJ programs are working in the US. The majority are not interested in pioneering MMJ in my area. It is FAR easier for physicians to hand out drug samples from the many reps that visit them each day. It's a no brainer to hand a patient a sample with dosage instructions already printed on the box, rather than try to work with patients to find a MMJ delivery method/strain/dosage that will help them. It's trial and error to find a strain, method etc that works and some doctors just don't want to be that involved. They're busy enough as it is.
And others really are just against it in general. For whatever their personal reasons are. I think as more patient experiences come out and more research is done, they will have to pay more attention to it as a treatment option for many, many conditions.
It reminds me of experiences I had with my thyroid medicine. I am on a natural desiccated prescribed thyroid medicine and in interviewing endocrinologists over the years I've had two of them tell me that they will never, ever prescribe it to patients. They prefer the Big Pharma synthetic drugs. There is research on my medication and it's one that had been used regularly before the synthetic versions were created, but these doctors put all their stock in "lab created, regulated medicine" and won't even look into anything else.