The present invention includes a co-crystal of creatine and citric acid obtained by milling, methods of making the same that include a process for the preparation of co-amorphous formulation of creatine and citric acid, the process comprising: mixing creatine and citric acid; milling the creatine and citric acid for a time sufficient to form a co-amorphous solid, which co-amorphous solid can be converted into a co-crystal, and nutritional supplements that include the same.