To the Editor.— As with many other clinical features of cannabis that were submerged until recently, the observation that regular use might have a cumulative effect requiring a period of abstinence for the symptoms to clear (222:35, 1972) goes back many years. Rosenthal translates from original manuscripts the medieval Moslem recorder Az-Zarkashi as stating the following of a certain Shayk Ali al-Hariri, a 13th century religious leader: This Hariri was very hard on habitual users of hashish. One of his followers sent a messenger to him to upbraid him (for his attitude). The Shaykh said to the messenger: "If the man mentioned is one of my followers, so that I have to oblige him, let him give up hashish for 40 days until his body is free from it, and 40 more days until he is rested from it, after having become free; then let him come to me so