Many people, myself included, would applaud you for beating opiate addiction. Unfortunately, as you are aware, many people aren't understanding. They just see you as a time bomb waiting to turn into a relapsed addict. Disclosing this can lead to being let go, being watched more closely than others, and if it gets out, being blacklisted. Don't do it. If I understand correctly you are not obligated to disclose this so you are protected in that regard if it does come out later.
Also ask yourself, Why does my employer need this? The only even slight possible reason they could have for legitimately needing this is in the exceedingly slim chance that there is an emergency that requires life-saving intervention and they need to be concerned with drug interactions.
What makes you not disclosing it different from every other employer I have ever had who don't have any medical history on me or any of their employees? Don't risk a high chance of discrimination for a infinitesimally small risk that you will be unconsciously administered a dangerous drug interaction. Furthermore, as pointed out in a comment by @dwizum, you can take responsibility for informing first responders with a bracelet or dogtag designed for such.
Do not indicate that you are refusing to fill it in as that only raises suspicion that you are indeed hiding something.