What is the religious ruling on neglecting one's health by not taking medication and not heeding doctors' advice? And what should one do who was unable to make up missed fasting days due to a deterioration in their health resulting from this neglect, with another Ramadan fast approaching?

The majority of scholars permit medical treatment, and some consider it recommended. It becomes obligatory if abstaining from it leads to death, due to Allah's prohibition against killing oneself: "And do not kill yourselves. Indeed, Allah is ever Merciful to you." A diabetic person may break their fast if fasting would harm them, and they must make up those days. If they are unable to make up the days before the next Ramadan, they must pay a compensatory expiation for delaying (a mudd of food for each day). If their inability to make up the days persists, then no expiation is required of them.