For many years it has been observed that individuals differ in their tolerance of depressant drugs. In everyday life differential susceptibility to the effects of alcohol is that most commonly observed and this fact led to McDougall's early theorizing (1929) about drug tolerance and personality. Also at this time Pavlov, in dogs, recorded differences in drug sensitivity in his various temperamental types. However, the first attempt to measure individual differences in drug tolerance was made by Shagass (1954), who proposed the concept of “sedation threshold”. This was defined in terms of the amount of sodium amytal required to bring about certain behavioural and other changes in the individual, the threshold itself lying somewhere between the state of complete wakefulness and that of complete sedation. Two methods of determining the threshold were employed by Shagass. First, the point of onset of slurred speech gave an approximate estimate of its position. Secondly, a more accurate determination was made by means of EEG changes, specifically the point at which inflexion occurs in the amplitude curve of induced fast frontal activity.

Fentazin—Perphenazine—is a phenothiazine derivative which has gained a considerable amount of popularity with psychiatrists employed in treating severe schizophrenia and chronic psychosis. At present, however, there would appear to be few reports of its use in mental deficiency practice.