A blood sugar level of 300 is considered too high and may lead to extreme thirst, increased appetite, weight loss or fatigue, according to WebMD. If blood sugar levels continue to increase above 350 milligrams per deciliter, a patient may become unconscious or lethargic.
Mild high blood sugar is described as a blood sugar level between 200 and 350 milligrams per deciliter, explains WebMD. A blood sugar level higher than 350 milligrams per deciliter is a more serious problem that can lead to more severe symptoms such as blurry vision, drowsiness, lightheadedness, vomiting, rapid breathing and a weak pulse. Patients with high blood sugar that continues to rise may lose consciousness, become confused or act extremely lethargic. Diabetics can avoid high blood sugar levels by taking their insulin or diabetes pills exactly as recommended. When high blood sugar occurs in spite of proper medication use, a patient should contact his doctor immediately. A change in medication or meal plans may be required.
Even though diabetics may not show any obvious symptoms, a sustained high blood sugar level has a number of long-lasting effects on the body, explains Mayo Clinic. Blood sugar coats the red blood cells, making them sticky and allowing cholesterol to build up on the insides of blood vessels, increasing the risk of heart attack and stroke. High blood sugar also leads to eye problems, kidney damage, gum disease and nerve damage in many patients.