Yeah 250 mg shouldn't give you the runs. They do use large doses to treat constipation, but if you took grams and grams of it it could be really harmful.
I think it does help but it's subtle. Anyway, in normal doses it doesn't hurt and is actually good for a lot of other things.
This is from WebMD:
"Some people use magnesium for diseases of the heart and blood vessels including chest pain, irregular heartbeat, high blood pressure, high levels of "bad" cholesterol called low-density lipoprotein (LDL) cholesterol, low levels of "good" cholesterol called high-density lipoprotein (HDL) cholesterol, heart valve disease (mitral valve prolapse), metabolic syndrome, clogged arteries (coronary artery disease), stroke, and heart attack.
Magnesium is also used for treating attention deficit-hyperactivity disorder (ADHD), anxiety, chronic fatigue syndrome (CFS), Lyme disease, fibromyalgia, cystic fibrosis, alcoholism, mania, recovery after surgery, leg cramps at night and during pregnancy, diabetes, kidney stones, migraine headaches, a long-term pain condition called complex regional pain syndrome, weak bones (osteoporosis), premenstrual syndrome (PMS), altitude sickness, urinary incontinence, a condition that causes burning pain and redness called erythromelalgia, restless leg syndrome, asthma, hayfever, multiple sclerosis, and for preventing hearing loss and cancer.
Athletes sometimes use magnesium to increase energy and endurance."