Creatine can cause side effects ranging from mild to extremely serious and dangerous. Creatine can cause nausea, cramping and an upset stomach. Since creatine absorbs water from cells, which therefore raises the water mass of the muscles, it can also cause dehydration and obesity. Creatine dehydration can also lead to depression and anger issues. Some more serious creatine side effects include kidney and gastrointestinal disorders, as well as severe anxiety, acne and even male breast development.