Definition of Posology
Posology is derived from the Greek words "posos" meaning how much and "logos" meaning science.
It refers to the study of drug dosages, which includes not only the amount of the drug to be administered but also the frequency and duration of treatment.
Posology aims to determine the correct dosage to achieve the desired therapeutic effect while minimizing adverse effects.
Factors Affecting posology
Several factors can affect posology, which is the determination of the appropriate drug dosages for a particular patient.
1. Age:
Drug metabolism varies with age. Infants and the elderly often require adjusted dosages due to differences in liver and kidney function, body composition, and metabolic rates.
2. Sex:
Biological differences, including hormonal variations between males and females, can influence drug metabolism and response, affecting dosing.
3. Body Weight:
Heavier patients generally require larger doses, while underweight individuals may need smaller doses to achieve the same therapeutic effect.
4. Route of Administration:
The method by which a drug is administered (oral, intravenous, inhalation, etc.) affects its absorption, distribution, metabolism, and excretion, thus influencing the dosage.
5. Time of Administration:
The timing of drug administration can impact its effectiveness, especially for drugs with specific circadian action or those interacting with food.
6. Environmental Factors:
External factors like temperature, humidity, and altitude can influence drug absorption and metabolism.
7. Emotional Factors:
Stress and psychological state can affect drug metabolism and the body's response to medications.
8. Presence of Disease:
Conditions like liver and kidney diseases can affect drug metabolism and excretion, necessitating dose adjustments.
9. Accumulation:
The buildup of a drug in the body, especially with repeated dosing, can lead to increased effects or toxicity, requiring careful dose management.
10. Additive Effects:
When two drugs with similar pharmacological effects are taken together, their combined effect can be additive, necessitating dose adjustments to avoid excessive response.
11. Synergism:
This occurs when the combined effect of two drugs is greater than the sum of their individual effects, often requiring dose reduction to prevent toxicity.
12. Antagonism:
when one drug reduces or opposes the action of another. Understanding this interaction is crucial for determining the appropriate dosages.
13. Idiosyncrasy:
Some patients may experience unpredictable and unusual drug reactions that are not related to the drug’s dose or mechanism of action, often due to genetic factors.
14. Tolerance:
Over time, some patients may develop a decreased response to a drug, requiring an increased dose to achieve the same therapeutic effect.
15. Tachyphylaxis:
A rapid decrease in drug response after repeated administration over a short period can occur, leading to the need for a modified dosing regimen.
16. Metabolic Disturbance:
Changes in metabolic rate (e.g., in conditions like hyperthyroidism or hypothyroidism) can affect drug metabolism and dosage requirements.