22 MATERIA MEDIC A AND THERAPEUTICS. 



(a) Many drugs have different actions in different 

 doses, which must be arranged accordingly ; e.g. anti- 

 monium tartaratum, alcohol, opium, and rhubarb. 



(b) The dose must vary with the age of the 

 patient, children getting but a fraction of a dose for 

 an adult. A convenient method of calculating the 

 doses for children under twelve, is to divide the age in 

 years by the age in years + 12, and to use the result 

 as the proper fraction of an adult dose. Thus, for a 

 child of four years the dose will be . 4 , , = ^ = \ of an 



4 + 12 lo 4 



adult dose ; for a child of twelve, j2Ti2 = S = 2 

 Above twelve, and under twenty-one, the dose must 

 lie between J and a full dose. Delicate persons and 

 patients exhausted by disease resemble children in 

 bearing but small doses. 



(c) In particular diseases the ordinary dose may 

 have to be modified. In disease of the kidneys, 

 where excretion is diminished, drugs which are dis- 

 charged by this channel, such as morphia, are retained 

 in the blood for a longer time, i.e. in larger quantity at 

 any given time after administration, and symptoms of 

 poisoning very readily supervene. Quite a different 

 matter is the effect of a disease in neutralising the 

 effect of a drug given to combat it. Thus, large doses 

 of morphia will be tolerated in severe pain, because 

 the action of the morphia is spent in overcoming the 

 pain. The periods of menstruation, pregnancy, and 

 lactation also require to be considered in prescribing. 



4. Frequency. Medicines are ordered to be taken 

 one or more times, according to the desired end. 

 Thus purgatives are generally taken in a single dose ; 

 an emetic is to be taken once, and repeated only in 

 case vomiting is not induced ; whilst tonics are 

 generally ordered three times a day continuously. 



5. Duration. The period for which a drug may be 

 given depends entirely on a variety of circumstances 



