What is the ruling on consuming tea and coffee—despite their harm—and eating sweets for a diabetic, by analogy with the prohibition of cigarettes due to harm?

The basic principle concerning things is permissibility until prohibition is established. If harm from using a substance is proven for an individual, based on a doctor's statement or personal experience, then it becomes obligatory for them to abstain from it, due to the hadith: "There should be no harm or reciprocal harm," and "The origin of everything harmful is prohibition."