Yes, Alfalfa tonic is generally safe and commonly used to stimulate appetite. However, a tonic alone is not enough for healthy weight gain. At 6 feet tall and 60 kg, you are underweight, but this can be safely managed. To gain weight healthily, focus on a high-calorie, nutrient-dense diet rich in proteins and healthy fats, alongside strength training. It is highly advisable to consult a healthcare provider or nutritionist first to rule out any underlying medical conditions and plan a structured diet.