You can have breakfast when you donate blood, and you must not donate blood on an empty stomach, which means you cannot donate blood on an empty stomach. If you donate blood on an empty stomach, some people may experience hypoglycemia, resulting in panic, sweating, chest tightness and a series of other uncomfortable symptoms, or in severe cases, hypoglycemic coma, manifesting as confusion and twitching of the limbs, which may endanger the health of the donor. Of course, there are also requirements in terms of diet before blood donation. Try to avoid high-calorie, high-sugar and high-fat foods, because after eating these foods, the blood viscosity will easily increase, and the lipid content in the blood will increase, which will easily show a thick blood color and affect the quality of the blood. Therefore, you must eat before donating blood, but pay attention to a light diet.