I would not assign the code just based on what you've said here.
I have never assigned Z79.899 to report simply that a patient is on a medication. I feel it adds no important information, and in some practices it would result in the code being used on almost every patient's visit. I only use this code if the provider has documented something specifically about the long-term use of a medication and how it is affecting the patient's care or treatment plan, for example if the patient's use of the medication is putting them at some kind of risk, or requires some kind of monitoring, or is incompatible with another medication that the provider would normally use to treat a problem - something to that effect.
So in situations where medication use is affecting the provider's decision making, I'll report it to show that it's a factor in the encounter, but I don't use it for every patient who is on a medication. So far, I've never had any issues with an auditor citing me an error for omitting the code.