It’s not neccesarily a myth. It’s usually fine for people to drink it. But some can be lactose intolerant. And my cf docs have always told me to refrain from drinking milk when I have an upper respiratory exacerbation.
When I don’t have an infection I drink milk every day for the calcium and vit d. Its a very digestible way of getting vitamins. And I love dairy too. Good source of good calories too.
Just ask your doctor b/c I’ve often heard to refrain from dairy when there’s exacerbation.