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Factors Modifying Drug Effects

Overview

Responses to drugs vary significantly among individuals, reflecting differences in pharmacokinetics and pharmacodynamics. This variation occurs within different patients or even within the same patient across different time points.

Key Factors Influencing Drug Response

1. Pharmacokinetics: Different individuals handle drugs differently based on metabolism and excretion capacities.

2. Receptor Variation: The number and state of receptors or proteins coupled with drug action can vary, affecting response.

3. Neurogenic/Hormonal Influences: Hormonal levels and neurogenic tone can alter drug efficacy.

Specific Modifying Factors

• Body Weight/Size: Gives baseline for average drug doses; obese or very lean individuals may need weight-adjusted dosing. BSA calculation:
  $BSA = BW(Kg)^{0.425} imes Height(cm)^{0.725} imes 0.007184$

• Age:

  • Pediatrics: Doses often use formulas like Young’s $ext{Child dose} = rac{ ext{Age}}{ ext{Age} + 12} imes ext{adult dose}$ and Dilling’s $ext{Child dose} = rac{ ext{Age}}{20} imes ext{adult dose}$. Children exhibit physiological differences affecting drug metabolism (e.g., higher water content, immaturity in liver/kidneys).

  • Elderly: Dosing considerations include renal function decline, multiple medication use, and chronic health states.

• Gender Differences: As females generally weigh less, they may require lower dosages. Special precautions are needed during menstruation and pregnancy.

• Pregnancy: Physiological changes impact drug absorption (e.g., reduced GI motility), volume distribution, and metabolism.

• Diet and Food: Nutritional status can influence drug metabolism and absorption; specific foods may delay gastric emptying or interact with drug absorption.

• Species and Race Variability: Different species exhibit varied drug resistance; humans also show differences in drug response based on racial genetic factors.

• Route of Administration: Dosage varies significantly between I.V. and oral routes; e.g., magnesium sulfate's effects differ by administration route.

• Biorhythms: Time of drug administration can alter efficacy (e.g., corticosteroids typically taken in the morning).

• Psychological State: Patient beliefs and expectations significantly affect drug efficacy, particularly regarding centrally acting drugs.

• Disease States: Pathological conditions can create limitations in drug efficacy, influencing metabolism and elimination.

Interaction Between Drugs

• Cumulation: Over-administration relative to elimination can result in drug accumulation.

• Additivity, Synergism, Potentiation:

  • Additivity: Combined effect equals the sum of individual effects.

  • Synergism: Combined effect exceeds the sum of individual effects.

  • Potentiation: One drug enhances another's effects without an effect alone.

Interactive Example

• Drug A with 0 response plus Drug B with 3 response yields 5 in combination (potentiation). Drugs B and C yield 8 (additivity), while C and D yield 20 (synergism).