A fasting blood sugar of 40 mg/dL is dangerously low, even in a non-diabetic person, and can cause sweating, dizziness, weakness, confusion, fainting, or seizures. This is not something to treat with routine medicines at home without finding the cause. Immediately take fast-acting sugar such as glucose, fruit juice, sugar water, or sweets if you have symptoms. Causes can include prolonged fasting, alcohol use, hormonal problems, liver disease, severe infection, or certain medications. Please consult a physician urgently for evaluation and repeat testing. You may need tests such as repeat fasting sugar, HbA1c, liver function, kidney function, insulin levels, and hormonal assessment. Seek emergency care immediately if you become confused, unconscious, or unable to eat.