10 CIRCUMSTANCES MODIFYING THE ACTION OF DRUGS 



Drugs, as opium and alcohol, acting especially on the 

 nervous system, often excite in therapeutic doses, but de- 

 press and paralyze in toxic doses. Drugs, as digitalis^ 

 stimulating the heart in medicinal doses, usually depress 

 and paralyze the organ in poisonous doses. Many drugs 

 promoting urinary secretion, in ordinary doses, cause inflam- 

 mation and urinary suppression in large doses. The best 

 way to determine the dose of a drug is to estimate the 

 amount required for each pound of live weight. This only 

 applies to the samespeciesand to animals of ordinary build. 

 Fat is a comparatively inert tissue as far as the action of 

 drugs is concerned. In the case of young animals, and 

 of those either above or under the ordinary size of the 

 adult of any species, the dose should be proportioned — ac- 

 cording to weight — to the average dose for the adult animal 

 of that species. Thus, if the average weight of a horse i» 

 1000 pounds, the dose of any drug for a colt weighing 

 500 pounds would be half the usual dose for adult horses. 

 In a general way the dose for all animals from birth to a 

 few weeks old, is one-twentieth of that suitable for the 

 mature animal of the same species ; for yearlings, about one- 

 third of the adult dose. The dose recommended for dogs 

 is commonly the same as that given to man, but this rule 

 does not apply in the case of some powerful drugs (strych- 

 nine), where the dose should be adjusted to the weight, i.e., 

 so much per pound, live weight. 



It is impossible to calculate the dose for all domestic 

 animals as based on that for animals of one species, be- 

 cause the differences in anatomy and physiology modify the 

 actions of drugs in degree and kind, but the dose for sheep 

 is about one-fourth of that for the laiger ruminants. 



The repetition of a dose is determined to a consider- 

 able extent by the duration and rapidity of a drug's action. 

 Agents used for their immediate effect, as those relieving 

 pain and stimulating the circulation and respiration, are 

 repeated frequently till the desired effect is attained. 

 Medicines improving the condition of the digestion, blood 



