Intravenous fluid therapy is sometimes used in cases of dehydration. A friend told me about a friend of hers who went hiking in a desert area of Nevada. It was a hot day and the hiker apparently didn't bring enough fluids or he just overexerted.
When he returned, he called my friend and she took him to urgent care. He had all the symptoms of severe dehydration. They tried a solution of salts and sugars by mouth, but he didn't improve, so my friend took him to the hospital and they began intravenously replacing the lost fluid.
It was a successful treatment. Word of warning - avoid dehydration, use common sense!