With rapid weight loss and craving for sweet, the first thing to be ruled out is diabetes. Hence fasting blood sugar and blood sugar two hours after meal should be tested. Yes, drug abuse is a possibility and blood tests are needed to rule that out. There are many other causes of unexplained weight loss like worms in stool, depression, anorexia nervosa, AIDS, cancers especially colon cancers, drug abuse, infections and loss of appetite. Loss of appetite can again be due to certain medications, drug abuse, depression, AIDS, acute and chronic infections, cancers and hypothyroidism. Either she does not eat well or there is increased metabolism as in hyperthyroid states, HIV, cancer etc. Poor absorption as in irritable bowel syndromes and malabsorption syndromes too is possible. Malabsorption syndromes are seen in intestinal tuberculosis, HIV, tropical sprue, parasites in stool and Whipple’s Disease.
Do discuss this with your step-daughter's doctor and get her examined. Take care!