Prune Juice. Prune juice may stimulate bowel movements, but this would be a temporary weight loss only -- and not much at that. If you're routinely constipated, consider increasing veggies, leafy greens, whole fruits (not juice -- too high in sugars to use all the time!), and other kinds of fiber. Also plenty of fresh water daily. These will help keep your GI system in good running order.
Not really. I wouldn't look for one food or drink to be a weight loss answer. Rather it is important to focus on a healthy diet and physical activity. Drinking too much juice can mean too muh intake of the fruit sugar which is not helpful in weight loss. Focus on fresh vegetables, fruits, lean protein and healthy fats. Drink mostly water and not sweet beverages such as fruit juice, sodas, etc.