In a written statement to the U.S. Senate Committee on the Judiciary, the top opponent of decriminalizing marijuana, General Barry R. McCaffrey, Director of the Office of National Drug Control Policy (ONDCP), outlined the dangers of marijuana.

There is clear scientific evidence that marijuana is harmful to one's brain, heart, and lungs. It limits learning, memory, perception, judgment, and complex motor skills like those needed to drive a vehicle. It has been shown to damage motivation and interest in one's goals and activities. It can cause chronic coughing and bronchitis. In short, it is a very dangerous drug.

Source:

U.S. Senate Committee on the Judiciary (2 December 1996).