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Definition and Factors Affecting Posology

As a field of medicine, the study of posology focuses on the dosages or quantities of medicines that can be provided to achieve the desired results.
From the Greek "posos" and "logos" comes "posology." As a field of medicine, the study of posology focuses on the dosages or quantities of medicines that can be provided to achieve the desired pharmacological effects. Age, climate, weight, gender, and time of administration are all factors that affect the outcome.

For each indication, use the recommended dosage and mode of administration. How much to take? Amount of medication to be taken per day, such as 1 or 2 milligrams per kilogram or mg/m2, or every 6 hours.

Dose: Amount administered or consumed by a patient in order to get the desired medical result. A patient's therapeutic benefit should be maximized with the least amount of medication.

Factors affecting posology
Factors Affecting Posology
There are some factors that affect posology:
1. Age
2. Sex
3. Body Weight
4. Time of Administration
5. Body Surface Area
6. Route of Administration
7. Emotional factors
8. Accumulation
9. Environmental Factors
10. Presence of Disease
11. Additive effect
12. Idiosyncrasy
13. Synergism
14. Tachyphylaxis
15. Antagonism
16. Tolerance
17. Metabolic Disturbances
18. Drug Dependence/ Addiction

1. Age

  • As a result of their underdeveloped hepatic and renal functions, newborn infants (pediatrics) are particularly susceptible to some medications. Drugs accumulate in the body's tissues if they are not detoxified and eliminated.
  • Drug clearance may be slowed in the elderly (geriatric) due to a loss in renal and hepatic function, increasing the risk of drug build-up and eventual toxicity.
  • Age-related changes in target tissues and organs may also lead elderly persons to respond inappropriately to the typical dose of a medication.
2. Sex
  • It is possible that women do not react to medicines in the same manner as males. The use of medicine at these times requires special caution for pregnant and nursing women.
  • It takes longer for women to get sedated after taking morphine or barbiturate. During menstruation, pregnancy, and nursing, it is necessary to take extra measures while giving medicines.
  • Serious purgatives, antimalarials, and ergot alkaloids are prohibited during pregnancy.
  • The fetus is affected by alcohol, barbiturates and opioids when pregnant.
3. Body Weight
  • It is always given in milligrams per kilogram of body weight (mg/kg) in any situation.
  • This technique is also used for individuals weighing 50-100kg.
  • It is possible that obese adults, small children, and malnourished individuals will not receive this dosage. It should be dependent on the individual's body weight.
  • As a result, medication concentrations at the site of action depend on the amount of medicine administered in proportion to body size. Thin or obese adults may require a dose adjustment.
  • Based on body weight, determine the dosage of a medication for children.
  • Children's medication dose should be determined based on body weight rather than age.
4. Time of Administration
  • Drug absorption is slowed down by food in the stomach.
  • Unfilled stomachs allow for faster absorption of medicines.
  • When taken on an empty stomach, the drug's effectiveness may be diminished.
  • Iron, arsenic, and cod liver oil should always be taken after meals.
5. Body Surface Area
  • Many physiological systems and body surface area are closely related (BSA).
  • It is possible to calculate the surface area of a human using a nomogram comprised of scales for height, weight, and surface area.
  • Adult and child-specific nomograms are provided.
  • A person's height and weight are connected by a straight line, which intersects the surface area column.
6. Route of Administration
  • The therapeutic effectiveness of the medication is affected by its route of delivery.
  • Intravenously given drugs enter the bloodstream directly, resulting in the presence of the whole quantity provided in the bloodstream.
  • As a result of multiple physical, chemical and biological obstacles to their absorption, including interactions with stomach and intestinal contents, oral medications are seldom entirely absorbed.
  • Through the intravenous method, the medication's effect is rapid, and the risk of drug toxicity is higher.

7. Emotional Factor
  • Females are more emotional than males, therefore certain medicines require less dosage in order to get the desired effect.
  • Angina pectoris and bronchial asthma have been successfully treated with placebos, which are inert dose forms that physically mimic the genuine medication.
8. Accumulation
  • After repeated administration, medications that are slowly eliminated can build up to hazardous levels in the body, resulting in toxic symptoms. E.g., digitalis, emetine and heavy metals.
9. Environmental Factors
  • Stimulating medicines are more effective when taken in the daylight, whereas hypnotic drugs are less effective when used in the daytime.
  • This is due to the fact that darkness is a sedative. At night, hypnotics are more effective than during the daytime.
  • During the day, the amount of barbiturate necessary to induce sleep is significantly higher than that needed at night.
10. Presence of Disease
  • Patients with liver cirrhosis may experience exceptionally long-lasting effects from drugs such as barbiturates and chlorpromazine.
  • Because streptomycin is mostly eliminated via the kidney, people with renal failure may be at risk of toxicity.
11. Additive Effect
  • Additive impact occurs when the combined pharmacological activity of two or more medicines is equal to the total of their separate actions.
  • The treatment of asthma can involve the use of substances such as ephedrine and aminophylline.
12. Idiosyncrasy
  • Allergy is another name for idiosyncrasy. It is termed idiosyncrasy when a patient's response to a medication is distinct from its typical pharmacological effect.
  • Aspirin, for example, may induce gastrointestinal hemorrhage at modest doses.
  • Penicillin with sulphonamide, for example, can induce severe toxic effects in certain people.

13. Synergism
  • When two or more medicines are used together, their effects are enhanced as a result. As a result, a phenomenon known as synergism has developed.
  • Examples include a mixture of procaine and adrenaline that extends procaine's effect.
14. Tachyphylaxis
  • If a medication is delivered repeatedly, the cell receptors get blocked and the drug's pharmacological effect is reduced. Tachyphylaxis or acute tolerance is the term used to describe the occurrence of a reduced reaction that cannot be reversed by increasing the dose.
  • Due to tachyphylaxis, repeated doses of Ephedrine in the treatment of bronchial asthma, for example, may generate very little response.
15. Antagonism
  • Drug antagonism occurs when one drug's activity is countered by another drug's action on the same pharmacological system.
  • When acid poisoning is treated with milk of magnesia, the alkaline action of milk of magnesia neutralizes the effects of acid poisoning.
  1. Competitive/Reversible antagonism: Both agonists and antagonists attach to the same location on the cell membranes. Acetylcholine and atropine are two examples.
  2. Non- competitive/ Irreversible antagonism: Antagonists inactivate receptors, preventing the formation of an effector complex with an agonist. A combination of phenoxybenzamine and adrenaline acts on the -receptor in the brain.
  3. Physiological antagonism: Two receptors are occupied by an agonist and an antagonist, but their actions are opposing. Hepatotoxic substances include adrenaline (bronchodilation) and histamine (bronchoconstriction).
16. Tolerance
  • The capacity to withstand the effects of a drug, especially if it is developed via repeated usage.
  • It is typical for medicines, such as antihistaminic and narcotic analgesics, to cause tolerance.
  • Normal sensitivity can be restored by temporarily stopping the drug's administration.
  • When starting therapy, use the lowest effective dosage and avoid extended administration to limit the development of tolerability.
17. Metabolic Disturbances
  • It is possible that changes in water-electrolyte balance, acid-base equilibrium and body temperature may alter the effects of medicines.
  • Salicylates only lower the body temperature if the individual's body temperature has increased. They do not have antipyretic properties at all.

18. Drug Dependence/ Addiction
  • Euphoria; Tolerance; Dependence/Habituation
i) Physical Dependence: Tea, Nicotine
  • Rely on drugs in order to function.
  • Withdrawal syndrome can occur.
  • When a drug is abruptly stopped.
  1. Drug classes differ from one another.
  2. Unbalance is created by compensating processes.
ii) Psychological Dependence: LSD, Marijuana, Opiates
  • In addition, there is a behavioural dependency.
  • High incidence of drug use, drug desire, and tendency to relapse after ceasing usage.
  • Properties of drugs that enhance their effectiveness
Measurement Conversions for Liquids
1cc = 1 ml
5 ml = 1 tsp
15 ml = 1tbsp
30 ml = 1 oz
480 ml = 1pt
3785 ml = 1gal
3 tsp = 1 tbsp
2 tbsp = 1 oz
16 oz = 1 pt
2 pt = 1 qt
4 qt = 1 gals
1 L = 1000ml
Pt= pint
Gal= gallon
Tsp= Tea Spoon
Tbsp= Table Spoon
Oz= Ounce
Qt= quarter

Measurement Conversions for Solids
1 kg = 2.2 lbs
1 lb = 454gm
1 oz = 30 gm
16 oz = 1 lb
Lb= pound
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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