What happens to people who are on medication when they are cursed?
Well, it depends on what they’re on medication for!
Celeste is, despite her current disguise, an archangel. Her grace has healing powers much stronger than what Castiel shows in Supernatural. The last thing she wants is any of her little experiments to get sick and die.
The curse sends a victim into a coma for a week because their body has to adjust to its new size. Part of that is any illnesses they had will be cured. This doesn’t extend to any disabilities. Anyone that’s blind or deaf will remain blind or deaf. Mental health issues are not affected by the curse either. Infections can be deadly at that size as well.
However, sicknesses as deadly as cancer and asthma will be healed. Borrowers live naturally long and robust lives (barring any accidents), and they very rarely get sick because of the Grace inside of them. It keeps away illnesses.
Hope this answers your question!