I treat Doctors advice with a great deal of suspicion. Basically they follow guidance for prescribing and diagnosing as directed by the drug companies.
There's no money in food grade hydrogen peroxide or something like colloidal silver hence Doctors don't advocate it.. if they did and it went wrong the docs would be sued, if a corporate drug goes wrong the drug company is responsible, so you see why docs do it (big pharma gets sued all the timee).
I don't necessarily doubt what your doc says is true, but the products he will provide are just as bad or worse.
I have used doctor prescribed picolax or citrifleet for scope prep despite telling docs it makes me feel horrible, and the scope has not been clear. I've used hydrogen peroxide off my own back, felt much better and the scoping was much clearer.
I'm not saying I advocate the use of hydrogen peroxide enemas, but i will say I have used hydrogen peroxide and similar products many times for all sorts of things on my self/ family and pets and have found it to be safe (even in excess) and effective.
I just returned from a holiday in Cuba where I used it twice, once I came down with what felt like the flu, achy and bunged up with a real temperature. I drank some in water and went to bed, next day I was fine (I've been doing this for years! It's also great for avoiding hangovers). I also used it for cleaning a nasty looking wound that looked like it was getting infected. Again it cleared over night.
I'm not trying to convince others to use it in any particular way. But I will say I'm 100% confident any doctor who tells you there are no health benefits to using it is wrong. There's literally thousands of sites online advocating this stuff, none are making money from the advice, so what is the driver? It just works for so many things, and it's cheap!
Just my 2p worth..