According to Webster's New World Dictionary, methadone is also labeled as "drug".
On the other hand, I understand your comment. This is due to the fact that in English "drug" is used in two different contexts: it can mean 1) a medicine (which is the sense it is used here) or 2) a narcotic.
In order to avoid confusion, I think it would indeed be wise to label it as "medicine" instead of "drug".
Thank you for your message. Could you perhaps be more specific about why you think the definition is stigmatizing? I see that Peterdg assumes it is the word drug that you object to, but methadone is a drug, so I wonder if it is something else that's troubling you?