The national cost of prescriptions for drugs for use in medical practice in the United Kingdome has risen progressively from £35,000,000 in 1949 to well over £100,000,000 a year now (Dunlop, 1965). The number of drugs taken by patients is vast, and the variety of drugs available for prescription is considerable. Prescribed in order to produce a specific therapeutic effect, few drugs, if any, are wholly free from the potential of producing undesirable side-effects. These vary from the trivial to the lethal, from the transient to the permanent. Methods for their accurate detection, qualitative evaluation and risk identification are vital prerequisites to therapeutic safety and refinement. Experimental techniques, using detailed observation on large samples of patients being treated by drugs, have been developed recently. A monitoring system which scans for side-effects of drugs represents a means whereby prospective and cumulative data on the frequency of occurrence and nature of these side-effects can accrue. This is referred to as drug monitoring.