Ephedrine is a medication used to prevent low blood pressure during spinal anesthesia. It has also be used for asthma, narcolepsy, and obesity but is not the preferred treatment. It can be taken by mouth or by injection into a muscle, vein, or just under the skin. Onset with intravenous use is fast, while injection into a muscle can take 20 minutes, and by mouth can take an hour for effect. When given by injection it lasts about an hour and when taken by mouth it can last up to four hours.