Understanding how drugs interact and modify each other’s effects is critical for safe prescribing and therapeutic optimization.
- Additive Effect
- Definition: The combined effect of two drugs is equal to the sum of their individual effects.
- Example: Aspirin + Acetaminophen
- Permissive Effect
- Definition: The presence of substance A is required for the full effect of substance B.
- Example: Cortisol enhances catecholamine responsiveness
- Synergistic Effect
- Definition: The combined effect of two drugs is greater than the sum of their individual effects.
- Example: Clopidogrel + Aspirin
- Potentiation
- Definition: Drug B, which has no therapeutic effect on its own, enhances the effect of Drug A.
- Example: Carbidopa + Levodopa (Carbidopa prevents peripheral conversion of Levodopa)
- Antagonistic Effect
- Definition: The combined effect of two drugs is less than the sum of their individual effects.
- Example: Morphine + Naloxone
- Tachyphylaxis
- Definition: Acute decrease in response to a drug after initial or repeated administration.
- Example: Repeat use of intranasal decongestants (e.g., oxymetazoline) → reduced effect + rebound congestion
Summary Table
| Term | Definition | Example | Mnemonic |
|---|---|---|---|
| Additive | Effect of A + B = sum of individual effects | Aspirin + Acetaminophen | 2 + 2 = 4 |
| Permissive | The presence of A is required for B to work | Cortisol → catecholamines | “A allows B” |
| Synergistic | Effect of A + B > sum of individual effects | Clopidogrel + Aspirin | 2 + 2 > 4 |
| Potentiation | Drug B enhances the effect of A (B alone is inactive) | Carbidopa + Levodopa | 2 + 0 > 2 |
| Antagonistic | Effect of A + B < sum of individual effects | Morphine + Naloxone | 2 + 2 < 4 |
| Tachyphylaxis | Acute ↓ response to repeated drug | Oxymetazoline | Rebound effect |
Learning Objective:
Understand and distinguish additive, permissive, synergistic, potentiation, antagonistic effects, and tachyphylaxis, recognize clinical examples, and apply this knowledge to predict drug interactions and therapeutic outcomes.
