It really depends on what documentation you are coding here. You can only code the diagnosis that the provider has documented as the patient's condition at the time of the encounter. So T46.6X5A would be used if the patient was being treated for a reaction to the drug, with M79.1 as an additional code to describe the reaction, if that's what the provider says was the reaction. Using M79.1 alone would mean that the provider documented the patient's active condition at the encounter as a myalgia without further detail. But is sounds like you're saying that the patient is unable to take a statin because they've had a reaction in the past, so that would be reported as history, not as an active condition under treatment. If that's documented as an allergy to the medication, you could use the appropriate Z88 code for an allergy status to the drug. If you could share more of the documentation you're coding from, perhaps we could offer more help here.
Insurance reps shouldn't be telling you what to code, they usually aren't certified coders and likely have not even seen the documentation, so I wouldn't give much weight to what they're saying here.
