I haven't sent one in for some time, but I have had some accepted.
The Calgary Herald requires a written copy (mailed or hand-deliver to their mailbox). About once a week, they print instructions and name of editor and address of 'Letters-to-the-Editor.'
Give full name, address, phone and all that stuff.
They may call to verify.
Main problem- they edit, and may shift your viewpoint, without notifying you.

Toronto Globe and Mail- I sent one and it was accepted and printed as I wrote it. They require full address, etc. as well.
Must be printed copy, mailed. (They may have changed procedures since then).

Medical info might give them pause if not from a doctor, nurse or health official. Much junk advice and fakelore is out there and a good editor is wary. If they have a Health column editor, contact him, as already noted above.

Community papers print letters as filler. Often the luck of the draw.