Personally. If a Dr etc advised me to take it or anything else I would tend to follow that advice with or without an actual prescription unless say I had some niggling doubts for some reason and I would prefer a clear record of that advice.I was told by my GP, to just take 1x75mg per day. I insisted the aspirin be prescribed, so it appeared on my record. I hate taking meds, I refuse to take anything not prescribed.
If someone is a bit likely to be forgetful or in say a care home etc then yes an actual prescription might be of benefit to make intentions clear to all parties.

