WebMD states that creatine is most likely safe for the average individual as long as that individual follows the recommended dosage guidelines. While further studies are necessary, taking high doses of creatine orally is possibly unsafe.
The recommended dose, according to WebMD, is 20 grams daily for five days and 2 grams daily thereafter. Not enough studies have been conducted to determine whether or not creatine is safe to take while pregnant or nursing, so such individuals are advised to refrain from taking creatine. Those with diabetes or a kidney disease should also avoid using creatine. Nausea, diarrhea, stomach discomfort and muscle cramps are possible side effects of creatine.