POSOLOGY
Posology :- (Derived from the greek word Posos-how much, and logos science) It is the branch of pharmacology dealing with doses.
(Most important for pediatric)
Dose: Is the quantitative amount
administered or taken by a patient for the intended medicinal effect.
- The idea being to produce the optimum therapeutic effect in a particular patient with the lowest possible dose.
Factors affecting drug dosage:
-Age
-Body Weight
-Body Surface Area
-Sex
-Pathological State
-Tolerance
-Drug-Drug Interactions
-Time Of administration
-Route Of Administration
-Pharmaceutical dosage form and drug physical.
Age
• Newborn infants (pediatric) 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.
Body weight
• The official usual doses for drugs are considered suitable for 70 kg (150 pounds) individuals.
• The ratio between the amount of drug administered and the size of the body influences the drug concentration at the site of action. Therefore, drug dosage may require adjustment from the usual adult dose for abnormally lean or obese patients.
Body surface area
• A close relation exists between a large number of physiological processes and body surface area (BSA).
• The surface area of individuals may be determined from a nomogram composed of scales of height, weight and surface area.
• Two such nomograms are presented, one for adults and one for children.
• Surface area is indicated where a straight line drawn to connect the height and weight of an individual intersects the surface area column.
Sex
• Women are more susceptible to the effects of certain drugs than
are men.
• Pregnant women and nursing mothers should use medications only with the advise and under the guidance of their physician. Examples of drugs that are transported from the maternal to the fetal circulation e.g. alcohol, anesthet gases, barbiturates, anticoagulants, etc.
• Because of the undeveloped drug detoxification and excretion mechanisms present in the fetus, concentrations of drugs may reach a higher level in the fetus than in the maternal circulation. The transfer of drugs from the mother to the nursing infant through human milk may occur with various drugs with the drug effects becoming manifest in the infant.
Pathological state
• The effects of certain drugs may be modified by the pathological condition of the patient and must be considered in determining the dose.
• Warning and precautions are used in the drug labeling to alert the physician to certain restrictions of a particular drug. the use
Tolerance
The ability to endure the influence of a drug, particularly when acquired by a continued use of the substance.
Tolerance occurs commonly in such drugs e.g.antihistaminics, narcotic analgesics.
Normal sensitivity may be regained by suspending the drug administration for a period of time.
The development of tolerance can be minimized by initiating therapy with the lowest effective dose and avoiding prolonged administration.
Drug-Drug Interactions
• The effects of a drug may be modified by the concurrent administration of another drug.
• These drug-drug interactions are due to Chemical or physical interaction between drugs or alteration of the absorption, distribution, metabolism or excretion patterns of one of the drugs.
• The effects of drug-drug interactions may be beneficial, Detrimental.
Time of Administration
• The time at which a drug is administered sometimes influences dosage. This is specially true for oral therapy in relation to meals.
• Absorption proceeds more rapidly if the stomach and upper portions of the intestinal tract are free of food, and an amount of a drug that is effective when taken before a meal may be ineffective if administered during or after eating.
• Irritating drugs are better tolerated by the patient if food is present in the stomach to dilute the drug's concentration.
Route of administration:
Drugs administered intravenously enter the blood stream directly and thus the full amount administered is present in the blood. • In contrast, drugs administered orally are rarely fully absorbed due to the various physical, chemical and biologic barriers to their absorption, including interactions with the gastric and intestinal contents.
• Thus, a lesser parentral dose of a drug is required than the oral dose to achieve the same blood levels of drug.
Pharmaceutical dosage form and drug physical state:
Increasing the surface area of a drug by the 12/20 of its particle size has a significant effect on the rate of absorption, therefore, the dose can be minimized by reducing the particle size.
Thus, crystalline and amorphous forms of a drug shows a significant difference in the rate of absorption.
Dose: Is the quantitative amount
- The idea being to produce the optimum therapeutic effect in a particular patient with the lowest possible dose.
Factors affecting drug dosage:
-Age
-Body Weight
-Body Surface Area
-Sex
-Pathological State
-Tolerance
-Drug-Drug Interactions
-Time Of administration
-Route Of Administration
-Pharmaceutical dosage form and drug physical.
Age
Body weight
• The ratio between the amount of drug administered and the size of the body influences the drug concentration at the site of action. Therefore, drug dosage may require adjustment from the usual adult dose for abnormally lean or obese patients.
Body surface area
• A close relation exists between a large number of physiological processes and body surface area (BSA).
• The surface area of individuals may be determined from a nomogram composed of scales of height, weight and surface area.
• Two such nomograms are presented, one for adults and one for children.
• Surface area is indicated where a straight line drawn to connect the height and weight of an individual intersects the surface area column.
Sex
• Women are more susceptible to the effects of certain drugs than
are men.
• Pregnant women and nursing mothers should use medications only with the advise and under the guidance of their physician. Examples of drugs that are transported from the maternal to the fetal circulation e.g. alcohol, anesthet gases, barbiturates, anticoagulants, etc.
• Because of the undeveloped drug detoxification and excretion mechanisms present in the fetus, concentrations of drugs may reach a higher level in the fetus than in the maternal circulation. The transfer of drugs from the mother to the nursing infant through human milk may occur with various drugs with the drug effects becoming manifest in the infant.
Pathological state
• The effects of certain drugs may be modified by the pathological condition of the patient and must be considered in determining the dose.
• Warning and precautions are used in the drug labeling to alert the physician to certain restrictions of a particular drug. the use
Tolerance
The ability to endure the influence of a drug, particularly when acquired by a continued use of the substance.
Tolerance occurs commonly in such drugs e.g.antihistaminics, narcotic analgesics.
Normal sensitivity may be regained by suspending the drug administration for a period of time.
The development of tolerance can be minimized by initiating therapy with the lowest effective dose and avoiding prolonged administration.
Drug-Drug Interactions
• The effects of a drug may be modified by the concurrent administration of another drug.
• These drug-drug interactions are due to Chemical or physical interaction between drugs or alteration of the absorption, distribution, metabolism or excretion patterns of one of the drugs.
• The effects of drug-drug interactions may be beneficial, Detrimental.
Time of Administration
 |
• The time at which a drug is administered sometimes influences dosage. This is specially true for oral therapy in relation to meals.
• Absorption proceeds more rapidly if the stomach and upper portions of the intestinal tract are free of food, and an amount of a drug that is effective when taken before a meal may be ineffective if administered during or after eating.
• Irritating drugs are better tolerated by the patient if food is present in the stomach to dilute the drug's concentration.
Route of administration:
Drugs administered intravenously enter the blood stream directly and thus the full amount administered is present in the blood. • In contrast, drugs administered orally are rarely fully absorbed due to the various physical, chemical and biologic barriers to their absorption, including interactions with the gastric and intestinal contents.
• Thus, a lesser parentral dose of a drug is required than the oral dose to achieve the same blood levels of drug.
Pharmaceutical dosage form and drug physical state:
Increasing the surface area of a drug by the 12/20 of its particle size has a significant effect on the rate of absorption, therefore, the dose can be minimized by reducing the particle size.
Thus, crystalline and amorphous forms of a drug shows a significant difference in the rate of absorption.
Prescription (most imp)
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