Posology and Factors affecting to posology

Definition, Advantages and Disadvantages

1. POSOLOGY Ms. Chitralekha G. Therkar Assistant Professor Department of Pharmaceutics, Siddhivinayak College of Pharmacy, Warora

2. INTRO D UC T ION Posology is a branch of medical science which deals with dose and quantity of drugs which can be administered to a patient to get its pharmacological effect. Posology is derived from two Greek words “posos” meaning how much and “logos” meaning science . Posology is the study of how medicines are dosed as the dose of drug cannot be fixed very easily, it depends upon various factors including age, climate, weight, gender and time of administration. It includes dosage for each indication, each method, route of administration for appropriate dose recommendations, Frequency of dosing etc. Eg, mg, mg/kg, mg/m2 e.g. once or twice daily or every 6 hours.

3. 1 Age 2 Sex 3 Body weight 4 Route of Administration 5 Time of administration 6 Environmental factors 7 Emotional factors 8 Presence of disease 9 Accumulation 10 Additive effects 11 Synergism 12 Antagonism 13 Idiosyncrasy 14 Tolerance 15 Tachyphylaxis 16 Metabolic disturbance's 17 Hypersensitivity Factors affecting dose

4. 1. AGE The pharmacokinetics of many drugs changes with age. Newborn infants (pediatrics) are abnormally sensitive to certain drugs because of the immature state of their hepatic and renal function by which drugs are inactivated and eliminated from the body. Failure to detoxify and eliminate drugs results in their accumulation in the tissues to a toxic level. Whereas, elderly patients are more sensitive to some drug effect e.g. hypnotics which may produce confusion state in them. 2. Gender Women do not always respond to the action of drug in the same manner as it done in men. Special care should be taken when drugs are administered during menstruation, pregnancy and lactation. Eg, Aloes should be avoided during menstruation. And antimalarial drugs, ergot alkaloids are contraindicated during pregnancy also alcohol, barbiturate, narcotic drugs acts on foetus through placenta so it should be avoided. During lactation, morphine, tetracycline avoided because of its affect on babies.

5. 3. BODY WEIGH T The average dose is mentioned in terms of mg / kg body weight. Or another technique is used as a total single for an adult weighing between 50-100kg meaning it includes same dose dispensing for all body weights . However, the dose expressed in this fashion may not apply in case of obese patients, children and malnourished patients. It should be calculated according to body weight . 4. ROUTE OF ADMINISTRATION The effectiveness of any drug formulation is generally controlled by the route of administration . Intravenous doses of drug are usually smaller than the oral doses because it directly goes to blood stream without going through the first pass metabolism and various other eliminations. Intravenous route might enhance the chances of drug toxicity sometimes. Oral route is most commonly used route for drug administration because of advantages like self medication, easy administration and no pain generation during administration and so on.

6. 5. TIME OF ADMINISTARTION The presence of food in the stomach region can delay the absorption process of drug and the drug gets rapidly absorbed from the empty stomach. But it does not mean that all drugs will show much effective absorption when taken during or after meal. Iron, arsenic and cod liver oil should be given after meal and antacid drugs taken before meal . 6. ENVIROMENTAL FACTORS The amount of drug prescribed is also affected by the environment. Sedatives are the drugs consumed at night to induce sleep and treat insomnia but if taken in day time, the dose required will much higher than taken at night. Similarly, stimulant drugs are mostly prescribed to be consumed in day time, but if it has to be consumed at night, then the amount of dose have to elevate to higher amount. Other than day and night, the weather also affects the dose amount. Eg, Alcohol containing formulations are more tolerant in winter than in summer.

7. 7. Emotional factors The personality and behavior of a physician may influence the effect of drug especially for the drugs which are intended for use in any psychosomatic disorders . The females are more emotional than male and required less dose of certain drug. Inert dosage forms called placebos which resemble to the actual medicaments in the physical properties which are known to produce the therapeutic benefits in disease like angina pectoris and bronchial asthma. 8. PRESENCE OF DISEASE The drugs like barbiturates and chlorpromazine may produces unusual prolonged effect in patient having liver cirrhosis. Such as, streptomycin produce toxic effect on the patient their kidney do not work properly because streptomycin excreted through kidney.

8. 9 . AC C U M UL A T I ON Some drugs produces the toxic effect if it is repeatedly administered for long time . Eg, digitalis, emetine, heavy metals because these drugs excreted slowly. This occurs due to accumulative effect of the drug. 10. ADDITIVE EFFECT When two or more drugs administered together then it is equivalent to the addition of their individual pharmacological action and this phenomenon of addition is called as additive effect. Eg, ephedrine and aminophylline in the treatment of bronchial asthma. 11. SY N E R G I SM When desired therapeutic result needed is difficult to achieve with single drug at that time then two or more drugs are used in the combination to increase their action this phenomenon is called as synergism. Eg, Procaine and adrenaline combination, increase the duration of action of procaine.

9. 1 2 . A N T OG O N I S M When the action of one drug is opposed by the presence of other drug on the same physiological system is known as drug antagonism. The use of antagonistic response to drugs is valuable in the treatment of poisoning. Eg, milk of magnesia is given in acid poisoning where alkaline effect of milk of magnesia neutralize the effect of acid poisoning. When adrenaline and acetylcholine are given together, they neutralize the effect of each other due to antagonism because adrenaline is vasoconstrictor and acetylcholine is vasodilator. 13. Idiosyncrasy Idio s y ncra s y i s also call e d as an alle r g y . Drug idiosyncrasy refers to untoward reactions to drugs that occurs in a small fraction of patients and have no obvious relationship to dose or duration of therapy and generally liver is the frequent target for toxicity. An extraordina r y response to a drug which i s di f f erent f rom its pha r macolog i cal action i s called idiosyncrasy . Eg, small quantity of aspirin may cause gastric hemorrhage.

10. 1 4 . TOLERANCE When an unusually large dose of a drug is required to elicit an affect ordinarily which is produced by the normal therapeutic dose of the drug, the phenomenon is called as drug tolerance. Eg, smokers can tolerate nicotine, alcoholic can tolerate large quantity of alcohol. The drug tolerance is of two types , True tolerance is produced to oral and parenteral administration of the drug. Pseudo tolerance is produce only to oral route of administration. 15. TACHYPHYLAXIS When some drugs administered repeatedly at short intervals, the cell receptors get blocked and pharmacological response to that drug get decreased. The decreased response cannot be reversed by increasing the dose and this phenomenon is called tachyphylaxis. Eg, ephedrine given repeated dose at short intervals in the treatment of bronchial asthma may produces very less response due to tachyphylaxis.

11. 16. METABOLIC DISTURANCE Changes in water and electrolyte balance or acid base balance, body temperature and other physiological factor may modify the effect of drug. Eg, salicylates reduce s body temperature only if an individual has rise in the body temperature. They have no antipyretic effect if the body temperature is normal. 17. Hypersensitivity This type of behaviour of drugs can be due to frequent or known toxic effect of the drugs like antibiotics, vitamins or some proteinous substance. If any person is sensitized by the prior administration of the same drug, a minute dose of drug will produce allergic reactions.

12. Calculations of doses Doses proportionate to age – 1. Youngs Formula Dose for child – × Adult Dose 2. Dillings Formula Dose for child – × Adult Dose Doses proportionate to body weight – Clark’s formula Dose for child – × Adult Dose Or Dose for child – × Adult Dose  

13. Doses proportionate to surface area % Adult dose –  

14. veterinary doses Dose required for animals are higher in comparison to human beings because of body weight and size. Therefore it’s important to supply correct dose. Factors affecting on veterinary doses 1. Age 2. Sex 3. Body Weigh t 4 . R o u te o f A d m i n istrati o n 5 . T i m e o f A d m i n istrati o n 6. Environmental Factors 7. Habit 8. Rate of elimination 9. Effect of drug 10. Purpose of medication 11. Species 12. Character of drug

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