Milk, like other foods, can be safely reheated once. Any additional reheating can cause illness. Reheating foods and beverages also diminishes the quality.
Most foods should only be left out for a short period of time. After two hours, bacteria begins to grow. Reheating milk or another food can help kill some of the bacteria found in the food. When reheating, the item should reach 165 degrees Fahrenheit to kill any living bacteria. Repeated heating can cause the bacteria to become more resilient to heat, however. Staph is one bacterium that is known to grow in food that is not heated correctly.