When do medications become too old to take???

Two years ago a doctor gave me a sample pack of the sleeping pill Ambien to try. Then I got pregnant, and fed my daughter for a while and didn't take them. I still have some of the pills, but they are now 2 years old. I have had such trouble sleeping and would love to get a good night's rest. Are these pills to old to take?? Could they be harmful, or just ineffective?? Thanks!!