Core9: Therapeutic index

🧮 High-yield / 要点：Therapeutic index measures drug safety: TI = TD50 / ED50. Wider TI = safer drug.

Key definitions / 定義

• ED50：effective dose producing the desired response in 50% of the population.
• TD50：toxic dose producing toxicity in 50% of the population.
• Therapeutic index / TI：
  • TI = TD50 ÷ ED50
  • Larger gap between effective and toxic dose → safer drug.

Meaning / 意味

Concept	Meaning	Clinical implication
Wide TI	Toxic dose is much higher than effective dose	Relatively safe; easier dosing
Narrow TI	Toxic dose is close to effective dose	Needs careful dosing and monitoring

Examples / 例

• Lithium：very narrow therapeutic index; toxicity risk with small concentration changes.
• Digoxin / digitoxin：narrow TI; requires serum level and toxicity monitoring.
• Penicillins, macrolides：wide TI; generally much safer.

Related concepts / 関連概念

• Potency：less drug needed to cause a response → lower ED50/EC50.
• Efficacy：maximal biological effect a drug can produce.
• Therapeutic index is mainly about safety margin, not potency or efficacy alone.

Remember / 覚え方

• TI = Toxic / Effective
• Wide TI = wide safety window
• Narrow TI drugs need monitoring