Fire employee for positive cocaine test

Q. We are a small but growing construction company, and we don’t have formal policies in place. Recently, one of our employees was involved in an accident at a construction site. This is his third accident. After the second time, we had him sign a warning notice that said he’d be terminated if it happened again. We sent him for drug testing after this third accident and he came back positive for cocaine. We want to terminate him. But we suspended another worker who tested positive for marijuana. Can we fire him? —B.O., Pennsylvania