Chapter-3
Posology
( Syllabus: Posology- Dose and dosage of drugs, factors influencing dose, calculations of doses on the basis of age, sex, surface area and veterinary doses.)
• The word posology is derived from the Greek words 'posos' meaning how much and 'logos' meaning science. So 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 actions.
• The dose of a drug cannot be fixed rigidly because there are so many factors which influence the doses. These factors are age, condition of the patient, severity of the disease, tolerances both natural and acquired, idiosyncrasy, route of administration, formulation used, drug interactions and rate of elimination.
• The official doses (doses which are given in pharmacopoeia) represent the average range of quantities suitable for adults which administered orally within 24 hours.
• When other routes of administration are followed the relevant appropriate dose is given. It is the responsibility of the prescriber regarding the amount of the drug to be prescribed or the frequency at which the drug to be administered. But before dispensing any prescription, it becomes the duty of the pharmacist to satisfy himself that the overdose has not been prescribed. This can be confirmed either from the prescriber or by consulting the pharmacopoeia.
FACTORS INFLUENCING DOSE
1. Age: The age of an individual is of great significance. Children and old people need lesser amount of drug than the normal adult dose, because they are unable to excrete drugs as compare to adults.
2.Sex: Generally females require lesser dose than males because of their lesser weight and also due to the reason that they are more responsive to the effects of certain drugs than males, Drugs should be given very carefully during menstruation, pregnancy and lactation. Strong purgatives should be avoided during menstruation.
3. Body Weight: Generally recommended adult doses are based on a normal body weight of 70 kg. But such a dose will be too less for a muscular person weighing 100 kg and too large for a weak person weighing about 50 kg. The calculation of doses for children on the basis of body weight is considered more dependable than that based strictly on age.
4. Route of Administration: The dose of a given drug may vary according to the dosage form and route of administration used. Drugs administered intravenously enter the blood stream directly hence require lesser dose than the subcutancous dose which in turn is smaller than the oral dose.
5. Time of Administration: Time of administration of drugs is very important. Drugs are rapidly absorbed from the empty stomach, hence an amount of drug that is effective when taken before a meal may be ineffective if administered during or after meals, On the other hand iritating drugs are better tolerated if administered after meals which will dilute the drug's concentration and reduce the gastric irritation, e.g., iron, arsenic, cod-liver oil, etc., should always be given on a full stomach.
6. Presence of Disease : Drugs are more effective in diseased conditions than normal body conditions. During fever one can tolerate high doses of antipyretics than in a nonfebrile condition. Similarly during hepatic or renal disturbances, the drugs which are metabolised in the liver or excreted through the kidneys may prove fatal.
7. Environmental Factors: Alcohol is better tolerated in cold environments than in summer. Dose of a sedative required to produce sleep during day time is much more than the dose required to produce sleep during night.
8. Emotional Factors: Females are more emotional and responsive to drugs therefore require less dose of drugs, The faith inspired by the doctor on the mind of a patient is an important factor in medication. Nervous patients require smaller doses of drugs as compared to normal patients.
9.Accumulation: When a drug is repeatedly administered for a long time, depending on its nature, it may unexpectedly accumulate in the body to produce sudden toxic symptoms. The drugs which produce such symptoms of poisoing are called cumulative drugs.
10.Synergism:
When two or more drugs are used in the combined form their action is either increased or decreased depending on the drugs used in combination When the potency or duration of action is increased, the phenomenon is called synergism. This synergism is of two types :
(a) addition When the total effect of two drugs is just equal to the sum of their individual effects, it is known as addition.
(b) Potentiation When the combined effect of two drugs administered is greater than the sum of their individual effects, it is known as potentiation.
11.Antagonism: When the action of one drug is opposed by the other drug, the phenomenon is known as antagonism.
12. Habituation and Addiction
(a) Habituation When repeated use of a drug or agent leads to production of emotional or psychological dependence rather than compulsion the condition is known as habituation, e.g., use of tea, coffee, tobacco.
(b) Addiction: It is a state of psychic and physical drug dependence. Continuous use of alcobol, opium, cocaine, heroin, morphine, pethidine, LSD leads to addiction and turns the person to a wreck who becomes liability to society.
13.Idiosyncrasy
All persons do not respond alike to the same drug due to varied individual susceptibility, some may produce abnormal reaction to a drug. When an abnormal or unusual reaction is produced by a drug it is known as idiosyncrasy, e.g., few mg of aspirin may produce gastric haemorrhage and small doses of quinine may produce ringing in the ear.
14. Hypersensitivity
Hypersensitivity is an allergic reaction to a drug and is different from either the expected pharmacological response or toxic reaction to the drug. This is due to frequent or indiscriminate use of drugs like antibiotics, vitamins and especially proteinous substances. Once a person is sensitised, a minute dose of the drug will produce allergic reactions.
It is of two types
(i) immediate type which is serious and requires prompt injection of adrenaline othcrwise death may occur
(ii) delayed type in which urticaria, skin rashes or contact dermatitis may occur.
15. Tolerance
When a drug administered in an ordinary dose fails to produce the normal therapeutic effect and requires large dose of the drug to produce the normal effect. The unusual resistance thus produced is known as tolerance, e.g., smokers can tolerate nicotine, alcoholics can tolerate large doses of alcohol, rabbits can tolerate large doses of atropine due to quick destruction of the drug by enzyme atropine estrase present in their blood.
16.Tachyphylaxis
It is also known as acute tolerance. It is observed in certain drugs that when they are administered repeatedly at very short intervals the cell receptors get blocked up and pharmacological response to that particular drug is decreased. By increasing the dose this decreased response cannot be reversed. But if the administration of the drug is stopped for a long time and administered again after being discontinued then the initial effect of the drug can be reobserved. This condition is known as tachyphylaxis. Drugs like ephedrine, amphetamine, cocaine, and nitrites behave in this way.
CALCULATIONS OF DOSES
•Young's formula: Child dose = Age in years ∕ (Age in years+12) ✕ Adult dose
•Dilling's formula: Child dose=Age in years ∕ 20 ✕ adult dose
•Clark's formula: Child dose=Weight in pounds ∕ 150 ✕ adult dose Or
Child dose= Weight in kg ∕ 70 ✕ Adult dose
•Fried's formula: Child dose=Age in months ∕ 150 ✕ Adult dose
•Cowling's formula: Child dose=(Age in years+1) / 24 ✕ Adult dose
•Bastedo's formula: Child dose=(Age in years +3) / 30 ✕ Adult dose
•Catzel's formula: Child dose=Child's body surface area / Adult's average body surface area ✕ Adult dose
Short Questions & Answers
1. Young's formula is used for calculating the doses for children-
A. Above 12 years
B. Between 4 to 20 years
C. Between 12 to 25 years
D. Under 12 years
ANS: Under 12 years
2. Clark's formula is for calculation of the doses of a drug for children is based on-
A. Age
B. Drug activity
C. Body weight
D. Surface area
ANS: Body weight
3. Dose and frequency of administration of drug is determined by-
A. Dissolution rate
B. Disintegration rate
C. Half life of a drug
D. All of these
ANS: Half life of a drug
4. Which injectable route acts most rapidly?
A. Intramascular
B. Intravenous
C. Intradermal
D. Subcutaneous
ANS: Intravenous
5. Ampicillin suspension has been ordered for a child on the paediatric unit. The dose for the child is 250 mg t. i. d for 5 days. If one teaspoonful contains 250mg, what quantity of the suspension is required?
A. 10 ml
B. 75 ml
C.85 ml
D. 20 ml
6. Age at next birthday/24 *Adult dose is considered as-
A. Young's Rule
B. Cowling's Rule
C. Clark's formula
D. Dilling's formula
7. The rights of medication includes all of the following except-
A. Patient
B. Drug
C. Gender
D. Dose
8. A prescription calls for: Capsule Amoxycillin 500 mg PO t. i. d x 7 days. How many capsules of 500 mg will you dispense?
A. 42
B. 30
C. 7
D. 21
9. How many tablets of prednisolone (5mg) should be dispensed if it is written that 2tabs qid x 5days?
A. 44
B. 40
C. 10
D. 20
10. Intraocular refers to the treatment of the-
A. Eye
B. Heart
C.Lung
D. Ear
11. Child dose=(Age in years +3)/30 x adult dose
A. Bastedo's formula
B. Cowling's formula
C.Dilling's formula
D. Catzel's formula
12. If a physician orders for a 120mg dose of gentamycin 4Omg/ml, how much ml of the drug is required?
A. 80ml
B. 5ml
C. 60ml
D. 3ml
13. Characteristic unwanted reaction which is not related to a dose or to a pharmacodynamic property of a drug is called-
A. Tolerance
B. Tachyphylaxis
C. Idiosyncrasy
D. None of these
14. What does the term "potentiation" mean?
A. Cumulative ability of a drug
B. Fast tolerance developing
C.Intensive increase of drug effects due to their combination
D. Hypersensitivity to a drug
15. What phenomenon can occur in case of using a combination of drugs?
A. Tolerance
B.Tachyphylaxis
C. Synergism
D.Idiosyncrasy
16. If the adult dose of a drug is 60mg, then the dose of a 6years old boy should be-
A. 30mg
B. 20mg
C. 15mg
D. 10mg
17. The branch of medical science which deals with dose or quantity of drugs which can be administered to the patient is called as-
A. Rheology
B. Posology
C.Pharmacology
D. Physiology
18. Dose of a child of age 4 years as per Young's formula is-
A.1/3 of adult dose .
B. 3/4 of adult dose
C.1/ 4 of adult dose
D. 2/3 of adult dose
19. Which of the following formula of child dose calculation related to surface area?
A. Young's Rule
B. Clark's formula
C.Dilling's formula
D. Catzel's formula
20. Age in years/20 x Adult dose is known as-
A. Fried's formula
B. Dilling's formula
C. Cowling's formula
D. Clark's formula
Answer
5.one teaspoonful=5ml
5ml contain =250mg
5ml × 3times per day=15ml required
So, 15ml ×5days= 75ml (answer )
6.B. Cowling's Rule
7.Gender
8. 21
3times×7days=21capsule
9.40
2tables ×4times=8
8×5days=40
10.Eye
11. Bastedo's formula
12.3ml
1ml contain = 40mg
So, 120mg/40mg=3ml
13.C. Idiosyncrasy
14.C.Intensive increase of drug effects due to their combination
15.Synergism
16.20mg
Young's formula: Child dose=age in years/ (age in years+12) x Adult dose
17.Posology
18.1/ 4 of adult dose
19.Catzel's formula
20.Dilling's formula
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