PRESCRIBING. 23 



which need not be discussed here. We must refer, 

 however, to accumulation, toleration, custom, and 

 babit. When a drug is allowed to enter the system 

 at short intervals, for a sufficient period, more rapidly 

 than it can be excreted, a time will obviously come 

 when it will have accumulated so much in the tissues 

 as to produce its effects in a marked degree. Powerful 

 drugs, e.g. strychnia and digitalis, may thus begin 

 to act as poisons after having been given in the same 

 doses with benefit Jor weeks. On the other hand, 

 certain drugs lose their effect when given for a length 

 of time, from some cause still obscure, e.g. opium. 

 The dose must then be steadily increased, toleration 

 being said to be established by custom. If a patient 

 become dependent on a drug, crave for it, and indulge 

 in it to an unfortunate or even vicious extent, he is said 

 to have developed a habit for that drug, such as the 

 opium and alcohol habits or the habitual use of enemata. 



6. Time. The times of the day or night at which 

 the doses must be taken are of the first importance ; and 

 speaking generally, it may be said that every advantage 

 must be taken in this respect of the natural tendency 

 which it is desired to assist or stimulate by the drug. 

 Thus, drugs which induce sleep are naturally given at 

 bedtime ; alkaline stomachics before meals ; saline 

 purgatives early in the morning. The time required 

 by the drug to act must also be calculated, especially 

 in the case of the different purgatives. 



7. Combinations : Chemical and Physiological 

 Incompatibles. In most instances more than one 

 drug has to be given at the same time, and the prac- 

 titioner finds that he must combine them in a single 

 prescription, whether, for instance, pill, powder, 

 or liniment. Successful combination is at once the 

 most important and difficult part of the art of 

 prescribing. Whilst it affords the prescriber an op- 

 portunity of applying the whole of his knowledge of 



