Pharm 161 Posology Quiz

Pharm 161 Posology (Calculation of Doses)

• The course introduces posology, dose selection, and dose calculation for infants and children.
• Learning outcomes include defining posology, explaining factors affecting dose selection, and calculating doses for infants and children.
• Posology comes from the Greek words "posos" (how much) and "logos" (science).
• Posology is the branch of medicine that deals with doses.
• A dose is a prescribed amount of medicine taken at one time.

Factors Influencing Doses

• Genetic makeup: Variations in responses to the same dose due to genetic differences.
• Acquired Tolerance: Drug addicts can tolerate higher doses over time. Examples include morphine, heroin, and cocaine.
• Idiosyncrasy: Exceptional or individual intolerance towards certain drugs. Examples include aspirin causing asthma, penicillin causing rashes, and barbiturates causing measles-like skin reactions.
• Route of Administration: Absorption rate and completeness differ based on the method (e.g., injection vs. oral). Oral doses are typically larger than parenteral (injection) doses.
• Rate of Elimination: Duration of a drug's activity and potential for toxic reactions depend on this factor. Age or disease conditions can affect the liver's function, delaying drug breakdown. Reduced kidney activity can result in incomplete drug excretion.
• Drug Interactions: Concurrent administration of two drugs can lead to synergistic effects, requiring dose adjustments (reduction) for one or both drugs. Dangerous reactions can also occur. Use official books like BNF and BPC to find more about drug interactions.
• Formulation: Drug absorption and dose can be influenced by the drug's form (e.g., tablets, capsules).

Paediatric Dosage

• Doses can be adjusted based on age, body weight, or surface area. This is often necessary for children.
• Commonly used methods for adjusting for age include Young's Rule and Dilling's Rule.

Young's Rule

• Calculated using the formula: (Age in years / Age in years + 12). It gives the proportion of adult dose.
• Example: For a 6-year-old child, the proportion would be (6 / 6 + 12) = 6/18.

Dilling's Rule

• Calculated using the formula: (Age in years / 20). It gives the proportion of adult dose.
• Example:For a 6-year-old, it's (6 / 20).
• Doses are often proportionate to body weight or surface area. This is because drug concentration at the site of action depends on the individual's size.

Dose Frequency

• Dose frequency can be expressed as a daily or a 24-hour range as well as stated for different routes like oral, subcutaneous or intravenous.
• Appropriate for administration over 24 hours and should be divided doses.
• Example: Griseofulvin 1 g daily in divided doses.
• The quantities mentioned on official charts are often for a single dose, even if there isn't a specific statement.
• E.g., Sodium bicarbonate 5 g (single dose)
• Sometimes there's debate on administering doses more than once a day. Refer to the appropriate medical references or consult a prescriber/ pharmacist in such situations.

Official Doses

• Official doses are not always binding on the prescriber.
• If the doses need to exceed the official maxima then this needs to be underlined, circled, or initialled by the prescriber. It is the responsibility of the pharmacist to ensure that an overdose has not been prescribed.
• Pharmacists must double-check each dose before dispensing a prescription and be updated on changes in dosage recommendations.

Tutorial

• Refer to the British National Formulary (BNF) for further practice.