When is weight loss dangerous? Weight loss becomes dangerous when a person loses approximately 5% of their body weight within 6-12 months without following a specific diet or exercise program. It's worth noting that safe and healthy weight loss ranges from 0.45 to 0.9 kilograms per week when a person is following a diet or exercise program. What are the serious causes of weight loss? After answering the question "When is weight loss dangerous?", we must now explore the various reasons that lead to dangerous weight loss. Al-Mustaqbal University, the leading university in Iraq.