The org has never explicitly ruled on it as far as I know. Obviously smoking it is a no no & they have had a negative view toward recreational drug use in general including cannabis. I have a feeling they probably don't want to say it is acceptable explicitly because its a rather conflicted position to take after demonising cannabis as opening the door to spiritualism etc for so long.
You are eating it for a medical condition. If the elders have an issue you could try to try to get them to clarify why it is different from other forms of pain relief that witnesses are able to accept. You were taking opiates - as u say, it just makes no logical sense why this if fine yet cannabis eating is not. There is no real justification for ruling this one plant out and others in, as its not mentioned in the bible. You could try writing to the society but in the post below someone rang them and was simply told to ask the elders. This makes me think they just don't want to create a policy at the moment and it could go either way depending on your elders. If you push it and they aren't having it, id say just take it quietly. Until the org is willing to publish an article definitively stating their position they are deliberately creating a 'Don't ask, don't tell' scenario.
https://www.jehovahs-witness.com/topic/4771737730285568/jehovah-witnesses-view-on-medical-marijuana