Visual signs of long-term pot smoking are poor motor coordination, uncontrolled laughter, a lag or hesitation between thoughts, and unsteady hands. At one time, these were all thought to be short-term side effects--now known in many cases to be long-term with frequent use, according to a 1968 study by researchers W.H. McGlothin and L.J. West, published in the Hazelden booklet mentioned above. Other linked side effects include a symptom called amotivational syndrome, in which people become passive, apathetic, unmotivated, hedonistic, unconcerned about the future, unable to make plans and increasingly introverted.
You forgot to mention it will make white women want to sleep with black jazz musicians or lead to an explosion of teenage girls giving birth to frog babies.
"Now" known...? When did you write this, 1969?