What is the best prescription medicine for indoor mold allergies?

You mean allergies? Nasocorticosteroid sprays/antihistamine nasal sprays can be an alternative or addition to oral antihistamines. Prescription sprays include: flonase, nasonex, (mometasone) Veramyst and many others for the corticosteroid sprays. Astelin, patanase for the antihistamine sprays. Both need prescriptions. You can also do well with nondrowsy claritin/allegra or mildly sedating zyrtec otc.