Don't push it -- your health is too valuable to gamble with.
Getting lightheaded while you're on the treadmill is something you shouldn't ignore. While it's not likely to be a serious problem, you shouldn't take chances with your health -- especially when you're using a machine with moving parts that could hurt you or others. Take some steps to ensure you're protecting your health, and eventually that 15 minutes on the treadmill may be only the start of a healthy exercise routine.
Talk to Your Doctor
Feeling a little lightheaded after exercise is one thing, and may be due to the changes in blood pressure that happen during exercise. However, if you're feeling lightheaded during the workout itself, don't take any chances; talk to your doctor about the problem right away. She may recommend any number of tests to determine whether you have any heart problems that will stop you from exercising. If you're just starting out with exercise and you've been previously sedentary, you should always get your doctor's OK to start exercising anyway.
Water and Fuel
If your doctor doesn't find any serious issues or reasons why you can't continue exercising, look to other factors that may lead to that dizzy or lightheaded feeling. A big one for a lot of people is dehydration. Exercise causes a sudden drop in blood pressure, which, if you're dehydrated, can lead to dizziness. If you started the run in a dehydrated state, running is only going to make it worse. As a general rule, drink half your body weight in ounces every day. Also try drinking about 17 to 20 ounces two to three hours before you run, and about 8 ounces in the 30 minutes before the workout, suggests the American Council on Exercise. Likewise, eat a small snack such as a banana with almond butter or a granola bar before you work out to increase your blood glucose levels.
Warm Up and Cool Down
Speaking of that rapid drop in blood pressure, there's another thing you can do to mitigate its effects. Don't just start out running or walking fast on the treadmill. Instead, do a short warm-up of five to 10 minutes, walking slowly and allowing your heart to start beating faster gradually. At the end of your workout, repeat the process and cool down by walking or jogging slowly for five to 10 minutes.
Your Exertion Level
If you've just started a regular exercise routine, you may be experiencing symptoms due to your level of exertion. When you're pushing yourself hard, your body and brain may not be getting the oxygen they need, thus the dizzy, lighteaded feeling. It's a good thing that you're exercising -- but don't go overboard, especially at first. When you start feeling lightheaded, slow down. If you've set the treadmill at an incline, decrease it. You've already been on there for 15 minutes, so it's also not the worst thing in the world to end your workout now. Ideally, you should be exercising at a moderate pace for about 30 minutes, five days a week -- but you'll need to work up to that amount when you're just starting out.