Posology F. Y. B. Pharm Sem - I PDF

POSOLOGY F. Y. B. Pharm Sem – I Monal Jain  INTRODUCTION  Posology is derived from the greek words Posos- how much Logos- science  Posology is a branch of medical science which deals with dose or quantity of drugs which ca...

POSOLOGY F. Y. B. Pharm Sem – I Monal Jain  INTRODUCTION  Posology is derived from the greek words Posos- how much Logos- science  Posology is a branch of medical science which deals with dose or quantity of drugs which can be administered to a patient to get the desired pharmacological action.  The dose of a drug cannot be fixed rigidly because Various factors are responsible i.e. age, sex, severity of the disease etc.  The official doses in pharmacopoeia represent the average range of quantity suitable for adults which is administered orally within 24 hrs.  When other routes of administration are followed the relevant appropriate dose is given. FACTORS INFLUENCING DOSE 1. Age 2. Sex 3. Body Weight 4. Route of Administration 5. Time of Administration 6. Environmental Factor 7. Emotional Factor 8. Presence of Disease 9. Accumulation 10.Additive Effect 11.Synergism 12.Antagonism 13.Idiosyncrasy 14.Tolerance 15.Tachyphylaxis 16.Metabolic Disturbance  AGE The pharmacokinetics of many drugs changes with age. Newborn infants (paediatric) 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. Hence children wants less doses as compared to adult. Whereas, elderly patients are more sensitive to some drug effect e.g. Hypnotics which may produce confusion state in them.  SEX 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 & lactation. The strong purgative eg. Aloes should be avoided during menstruation. Similarly the drugs which may stimulate the uterine smooth muscles e.g. Drastic purgative, antimalarial drugs, ergot alkaloids are contra indicated during pregnancy. Alcohol, barbiturate, narcotic drugs acts on foetus through placenta. During lactation, morphine, tetracycline are avoided because it affect babies.  BODY WEIGHT The average dose is usually mentioned in terms of mg per kg body weight. Another technique used as a total single for an adult weighing between 50- 100kg. However, the dose expressed in this fashion may not apply in case of obese patients, children & malnourished patients. It should be calculated according to body weight.  ROUTE OF ADMINISTRATION I.V doses of drug are usually smaller than the oral doses, because intravenous route might enhance the chances of drug toxicity. The effectiveness of drug formulation is generally controlled by the route of administration. TIME OF ADMINISTRATION The presence of food in the stomach delay the absorption of drug & rapidly absorbed from the empty stomach. But it does not mean that drug is not effective when taken during or after meal. Iron, arsenic & cod-liver oil should be given after meal & antacid drugs should be taken before meal.  ENVIRONMENTAL FACTORS The personality & behaviour of a physician may influence the effect of drug especially the drugs which are intended for use in a psychosomatic disorders. The females are more emotional than male & requires less dose of certain drugs. Inert dosage forms called placebos which resemble the actual medicament in the physical properties are known to produce therapeutic benefit in disease like angina pectoris & bronchial asthma.  PRESENCE OF DISEASE If the patient has presence of any disease in the body, it may alter the overall effect of the drug. E.g. Drugs like barbiturates & chlorpromazine may produce unusually prolonged effect in patient having liver cirrhosis. E.g. streptomycin produces toxic effect in patient whose kidney function is not working properly, because streptomycin is majorly excreted through kidney. ACCUMULATION Some drugs produces the toxic effect if it is repeatedly administered for long period of time e.g. Digitalis, emetine, heavy metals because these drugs excreted slowly. This occurs due to accumulative effect of the drug.  ADDITIVE EFFECT When the pharmacological action of two or more drugs administered together is equivalent to the sum of their individual pharmacological action, the phenomenon is called as additive effect. E.g. ephedrine & aminophylline in the treatment of bronchial asthma. SYNERGISM When desired therapeutic result needed is difficult to achieve with single drug at that time two or more drugs are used in the combination form for increasing their action. This phenomenon is called as synergism. E.G. Ephedrine & adrenaline combination, increase the duration of action of procaine.  ANTAGONISM When the action of one drug is opposed by the 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. E.G. Milk of magnesia is given in acid poisoning where alkaline effect of milk of magnesia neutralise the effect of acid poisoning. When adrenaline & acetylcholine are given together, they neutralise the effect of each other due to antagonism because adrenaline is vasoconstrictor & acetylcholine is vasodilator.  IDIOSYNCRASY It is also called as allergy. An extraordinary response to a drug which is different from its characteristic pharmacological action is called idiosyncrasy. E.G. Small qty of aspirin may cause gastric haemorrhage. E.G some persons are sensitive to penicillin & sulphonamide because they produce severe toxic effect.  TOLERANCE When an unusually large dose of a drug is required to elicit an affect ordinarily produced by the normal therapeutic dose of the drug, the phenomenon is called as drug tolerance. E.g. Smokers can tolerate nicotine, alcoholic can tolerate large quantity of alcohol. The drug tolerance is of two types: true tolerance, which is produced by oral & parenteral administration of the drug. Pseudo tolerance, which is produced only to the oral route of administration.  TACHYPHYLAXIS When some drugs administered repeatedly at short intervals, the cell receptors get blocked up & pharmacological response to that drug decreases. The decreased response cannot be reversed by increasing the dose and this phenomenon is called tachyphylaxis or acute tolerance. E.G. Ephedrine, given in repeated doses at short intervals in the treatment of bronchial asthma may produce very less response due to tachyphylaxis.  METABOLIC DISTURBANCE Changes in water electrolyte balance & acid base balance, body temperature & other physiological factor may modify the effect of drug. E.g. Salicylates reduce body temperature only in case an individual has rise in body temperature. They have no antipyretic effect if the body temperature is normal.  CALCULATION OF DOSES The dose of a drug given in the pharmacopoeia represents the average maximum quantity of drugs which can be administered to an adult orally within 24 hrs. The doses are also calculated in proportionate to age, body weight & surface area of the patient.  DOSE PROPORTIONATE TO AGE There are number of methods by which the dose for a child can be calculated from the adult dose 1. Young’s formula 2. Dilling’s formula 3. Fried’s formula 4. Cowling’s formula 1. Young's formula : This formula is used for calculating the dose for children's under 12 years of age. Age in years Dose for the child = -------------------------------- X adult dose age in years + 12 2. Dilling’s formula: This formula is used for calculating the doses for children in between 4 to 20 years. This formula is considered better because it is easier & quick to calculate the dose. Age in years Dose for the child = --------------------------- x adult dose 20 3. FRIED’S FORMULA : This formula is used for calculating of dose for infants up to 2 years. Age in months Dose for infant’s= --------------------------- x adult dose 150 4. Cowling’s formula: Age at next birthday (in years) Dose for child= ------------------------------------------------x adult dose 24 DOSE PROPORTIONATE TO BODY WEIGHT Clark’s formula is used to calculate the dose on body weight. Childs weight in kg Dose for the child = -----------------------------------x adult dose 70 DOSE PROPORTIONATE TO BODY WEIGHT In this method the dose is calculated according to surface area. It is more satisfactory & appropriate method than methods based on age. Surface area of child Percentage of adult dose=------------------------------------- x 100 surface area of adult OR Catzel’s formula: surface area of patient in M2 Dose for patient =-------------------------------------------- x adult dose 1.73 M2 where, 1.73 M2 = average adult surface area.  PROBLEMS 1. Calculate the pediatric dose for a 6-year-old child if the adult dose is 300 mg (use Young’s Rule). 2. A child is 6 years old. Calculate the dose if the adult dose is 500 mg. 3. A 10-year-old child needs medication. If the adult dose is 300 mg, what is the child’s dose? 4. Calculate the dose for an infant aged 8 months, given that the adult dose is 200 mg. 5. An adult dose of a medication is 250 mg. Calculate the dose for a 6-month-old infant based on: Fried’s Rule. A child weighing 12 kg using Body Weight (dose = 15 mg/kg). A child with BSA = 0.65 m² using the BSA Formula. 6. An adult dose of a drug is 600 mg. Calculate the pediatric dose for: A 7-year-old child A child weighing 18 kg (dose = 8 mg/kg). A child with BSA = 1.0 m²