Sublingual B12 supplement causing pulmonary edema

You don't really know what is causing the fluid on your mum's lungs. Ask the doctor. (Has the doctor actually said she has fluid on her lungs, or have you "diagnosed" that yourself?)

A specialist is by definition going to know more about a specific area - the clue is in the title General Practitioner.
Are you medically qualified yourself?
Don’t you see that if pulmonary oedema is a consequence, your self medication may be doing real harm?

Your mum is most likely being treated perfectly well by her current doctor. If she needed a supplement the doctor would prescribe it. Or at least tell her to take it.

You should stop giving her anything OTC, and let her take her doctor's advice.

Do you get your B12 free?
Or do you buy it?
Thought so.

It's difficult to know what incentive a drug company would have in promoting B12. If anything, drug companies would be motivated to discredit the importance of any vitamin.
As for endocrinologists, I've heard that they vary in quality every bit as much as GPs. I hope I will be lucky and get a good one when I change doctors.

I’m all for being proactive but my heart sinks at studies have shown ....
Many “studies” are inconclusive and even worse, frequently funded by drug companies with their own agenda
A consultant endocrinologist is the right person to pronounce on your mother’s needs. This sounds dangerously like self- medication. Even vitamins can be harmful if they are not needed, particularly in the very young or the elderly.

If the cure is worse than the problem why use it
If a doctor is useless change doctors

Yes, my mum is being treated with carbimazole. There have been studies that show b12 is used up more quickly when you are hyperthyroid and when you are old you produce less intrinsic factor so you will absorb less. If you are younger and eat meat you could probably store 3-5 years worth of B12 in your liver, but someone of 89 might not have such a store to fall back on. I've applied to change my mum's doctor to someone more attentive and open minded.

I’m not sure why she would need B12 unless a deficiency has been diagnosed. I was hyperthyroid for years, on and off medication, but B12 was never mentioned as being needed. Better to treat thyroid condition.

A quick google has told me that taking Vitamin 12 can have serious side effects, which are probably going to more pronounced in someone your mother’s age..

If you’re worried that your mother is B12 deficient because of her hyperthyroidism, and the hyperthyroidism isn’t well controlled, and her GP isn’t helpful, I would suggest asking for a referral to a consultant endocrinologist.

Of course it could be that your doctor is ‘useless’ because your mother isn’t his patient and he couldn’t possibly comment!