Am I over-reacting to my son's behavior?

My son who just turned 16 went to a birthday party and came home late night intoxicated. We are not sure what substance he took but it could be anything from alcohol to nasty drugs. If it is alcohol I have no serious problems but what if he has taken drugs? He can be easily influenced by friends and I am very worried about what he did. I have decided to ground him for the next two years until he turns 18.