Posology
Definition:
Posology is derived from the Greek words posos meaning “how much” and logos meaning “science.” It is a branch of medical science that deals with the dose or quantity of drugs that can be administered to a patient to achieve the desired pharmacological action.
Factors Affecting Posology:
The following factors influence the dose of a drug:
- Age: Pharmacokinetics change with age. Children and elderly often require smaller doses since drug excretion ability varies. For example, elderly patients are more sensitive to hypnotics and tranquilizers which can cause confusion.
- Gender: Drug response may differ between men and women. Some drugs like morphine and barbiturates can cause more excitement in women before sedation. Special care is needed during menstruation, pregnancy, and lactation. Some drugs cross the placenta affecting the fetus.
- Body Weight: Dosages are often calculated per kilogram body weight. Average adult doses assume 50-100 kg weight but require adjustment in obese, malnourished, or pediatric patients.
- Route of Administration: Intravenous doses are smaller than oral due to direct bloodstream entry, making action faster and raising toxicity risk. Route controls drug effectiveness.
- Time of Administration: Food in the stomach can delay absorption. Drugs may work better on an empty stomach, while irritating drugs (e.g., iron, arsenic) should be taken after meals.
- Environmental Factors: Daylight stimulates and enhances stimulating drugs; darkness enhances hypnotics’ effects. Barbiturate doses needed for sleep vary by time of day. Alcohol tolerance varies by temperature.
- Emotional Factors: Physician’s personality and patient’s emotional state can affect drug response, especially in psychosomatic disorders. Females may require lower doses for some drugs.
- Presence of Disease: Diseases like liver cirrhosis or kidney impairment affect drug metabolism and excretion, requiring dose adjustments to avoid toxicity.
- Accumulation: Slowly excreted drugs can build up to toxic levels with repeated administration, e.g., digitalis, emetine.
- Additive Effect: Combined pharmacological action of two drugs equals the sum of individual effects, e.g., ephedrine and aminophylline for asthma.
- Synergism: Combined drugs produce an increased effect beyond additive, enhancing therapeutic outcomes.
- Antagonism: One drug opposes the action of another, useful in treatments like poisoning (e.g., milk of magnesia neutralizes acid poisoning).
- Idiosyncrasy: Unusual or abnormal drug response different from normal pharmacological reaction, often called drug allergy.
- Tolerance: Need for higher drug doses over time to achieve same effect, seen in smokers (nicotine) or alcoholics.
- Metabolic Disturbances: Changes in body’s water-electrolyte or acid-base balance affect drug action, e.g., salicylates reduce fever only if fever is present.
Detailed Notes:
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