This section is from the book "A Text-Book Of Materia Medica And Pharmacy For Medical Students", by Velyien Ewart Henderson. Also available from Amazon: A Text-Book of Materia Medica and Pharmacy for Medical Students.
The British Pharmacopoeia makes the following statement in regard to the doses as given in it. "The doses mentioned in the pharmacopoeia are intended to represent the average range in ordinary cases, for adults. They are meant for general guidance, but are not authoritatively enjoined by the council. The medical practitioner must act upon his own responsibility as to the doses of any therapeutic agents he may administer." This statement is a very important one and one that should be thoroughly understood by every medical practitioner. Firstly, the official doses represent the average range in ordinary cases. The deviations from the ordinary that are most likely to be met with must be considered. Weight. Roughly the larger and more robust the individual the larger the dose of most drugs that my be given to him. Small and weakly individuals should always receive small doses of any remedy at first. Sex. Women are often said to be less resistant to the action of drugs than men are but as a rule little distinction is made between the sexes. It must however be born in mind that at the time of pregnancy or menstruation any drugs that bring about changes in the blood-supply to the uterus or that would set up movements in its musculature should be either entirely avoided or given in very small doses and with caution. Also it must be remembered that many drugs are excreted in the milk and may readily make the milk unpalatable or even dangerous to a suckling child. Amongst the drugs excreted by the mammary glands are the oils of anise and dill, turpentine, copaiba, the purgative principles of rhubarb, senna, and castor oil, opium, iodine, also some of the metals antimony, arsenic, iron, lead, mercury and zinc. Idiosyncrasy. Every person differs from all others more or less. Each person is not only physically but also chemically a distinct individual. These personal differences are usually quantitively so small as to occasion little or no difficulty but occasional individuals are met with who deviate very widely from the normal in respect to some one or more drugs. Such individuals as are abnormally affected by any drug are said to have an idiosyncrasy for the drug. Drugs in regard to which idiosyncrasy is likely to be encountered are morphine, and its allies, mercury, bromides, copaiba, arsenic, iodides, quinine, etc. Idiosyncrasy is often an inherited characteristic. Tolerance. The continued use of a drug is very apt to make any individual less susceptible to its pharmacological action and to necessitate the administration of larger doses, this is known as tolerance. Tolerance often occurs with alcohol, morphine, arsenic, vegetable purgatives, cocaine.
Increased susceptibility to the action of the drug due to its continued administration also occurs. It rarely gives trouble except with those drugs such as Digitalis which can be more readily absorbed by the normal body than they can be excreted by it. Disease may readily influence the absorption of a drug on the one hand or interfere with its excretion on the other. For example a large skin-wound may readily absorb a poisonous dose of carbolic acid or iodoform. Or increased acidity in the stomach may lead to a larger absorption of bismuth salts than is normally the case. Diminished excretion by the kidney will lead to a more prolonged action of strychnine.
Secondly, the doses of the pharmacopoeia are doses for adults. For children much smaller doses must be given. The rule suggested by Young is perhaps the best for calculating the dose for a child. Multiply the adult dose by the age of the child and divide by the age of the child plus 12. Thus for a child of three, the dose would be 3/(3+12) or 1/5th; for an adult dose of 15 min. it would be 5 x (3/3+12) or 3 min. Another rule suggested by Brunton is to multiply the age at the next birthday by the dose and divide by 25 (the assumed adult age), or perhaps better multiply the dose by four times the age at the next birthday and divided by 100: for the example stated above that would be 4x4x15/100 or 2.4 min. roughly 2 1/2 min. Young children are particularly prone to be affected by morphine and its allied drugs, but are proportionately little influenced by strychnine, and alcohol.
Persons above the age of sixty are proportionately more affected by drugs than are younger persons, so that by adults must be understood persons between 20-60 years of age. Persons over 60 should receive roughly 3/4 and persons over 85 roughly 1/2 of the adult dose, save in the case of purgatives to which the aged are often very refractory.
Thirdly, the frequency of repetition makes a great difference in the size of dose to be administered. The more frequently the drug is to be administered the smaller the dose should be.
Fourthly, the time of day makes as a rule but little difference except with the case of drugs meant to bring on or increase a normal daily condition. For example a larger dose of a hypnotic such as chloral would be necessary to produce sleep during the day than at night. Also purgatives can best be given at such an hour that they will take effect at the hour of the patient's daily defecation. For this purpose calomel and aloes must be given some eight hours in advance, while purgative salts act within an hour or so.
The presence or absence of food in the stomach makes a great difference in the rapidity with which drugs are absorbed and in the quantity coming in contact with the wall of the stomach and so irritating it, and as a consequence of this larger quantities of any drug irritant to the stomach may be given immediately after than before meals.
Fifthly, Synergists are drugs having the same pharmacological final effects though the manner of action may be slightly different. For example, colocynth, aloes and potassium sulphate are synergists, as they are all purgatives. All of these occur in the Compound Pill of Colocynth. It is often an advantage, and this is especially the case in the administration of purgatives to include in a prescription two or more synergists. As in the instance mentioned above, when synergists are administered together it is necessary to give any one drug in only a fraction of its full dose.
Finally, the pharmacopaeial doses are not enjoined and the practitioner must use his own judgment. In many cases it is quite allowable to exceed the pharmacopoeial dose if the effects wished for are not achieved by its administration and the physician should carefully watch each and every patient and convince himself that the drugs given are really producing the wished for action. In other words he must not take it for granted that because he gives a pharmacopoeial dose of any drug that he must as a consequence get the described pharmacological action.
The doses of the pharmacopoeia are usually, and unless otherwise is stated, for administration by the mouth. Many drugs can however with advantage be administered by hypodermic, intramuscular or even by intravenous injection. Owing to the more rapid absorption as a rule of drugs given by these methods and to the certainty that they will be absorbed in their entirety it is not necessary that such large doses be given. In those cases in which drugs have given by intravenous injection only a small fraction of the dose given by the mouth was used. For drugs given sub -cutaneously (hypodermically) about one half of the dose is given that would be used if given by the mouth. Drugs given by inunction must be given in larger doses than would be used if they were given by the mouth. The same is true as a rule for drugs given by the rectum if they are intended to have a general action.