Yes. Lauki juice and karela juice are definitely good for fast weight loss. But also keep in mind that the juices of lauki and karela will not suffice. Along with it, you also need to have a balanced intake of calcium, protein, carbohydrates and iron. Drinking the juice of only karela and lauki will make your body very weak over a point of time. Thus you may not have sufficient energy to perform your day to day activities. However, it has been noted that karela juice is very beneficial for healthy hair and skin. So try and maintain a balance in your diet for weight loss.