Glytactin Ready to Drink 15 Chocolate 8.3 fl oz, 210 calories per carton. For the dietary management of phenylketonuria (PKU). Dispensed by prescription. Glytactin RTD Chocolate is a ready-to-drink metabolic formula product for PKU patients.Formula changes. Protein source is now GMPure (lower amount of phenylalanine/g protein equivalent (PE) 1mg/g protein vs 1.8mg/g protein). Enhanced vitamin and mineral blend.