Feeling comfortable with this statement is a key skill in general practice. Not every doctor finds it easy to say, but the more one can state it, the more one can use it as a career planning or appraisal tool.
“I don’t know” is a useful phrase. Whether patients like it or not is another issue – some prefer the doc to know everything and to be directed and others perhaps gain more trust with someone who admits when things are not entirely clear.
To use “I don’t know” (IDK) as a learning tool, start to note down the times you say you don’t know in a day, if any (itself an exercise in mindfulness); what it is about, and rank the level of comfort with this (e.g. 1 – 5, with 1 being very at ease and 5 being extremely concerned). It can sometimes uncover an area worthy of a CPD day or some extra reading.