Definitions & Distinction - Speedy Slowdown Showdown
| Feature | Tolerance | Tachyphylaxis |
|---|---|---|
| Onset | Gradual (days-weeks) | Rapid (mins-hrs) |
| Mechanism | PK/PD changes (receptor ↓) | Receptor desens., mediator depletion |
| Dose Effect | ↑ Dose may restore effect | ↑ Dose ineffective |
| Recovery | Slow (on withdrawal) | Rapid (drug holiday) |
| Examples | Opioids, BZDs, alcohol | Ephedrine, nitrates (IV) |
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Mechanisms of Tolerance - The Body's Adaptation
- Pharmacokinetic (Dispositional) Tolerance: Altered drug ADME.
- ↑ Metabolism: e.g., CYP450 enzyme induction (barbiturates, rifampicin).
- ↓ Absorption or ↑ excretion.
- Result: ↓ plasma drug concentration, ↓ therapeutic effect.
- Pharmacodynamic (Cellular) Tolerance: Altered drug-target interaction.
- Receptor Changes:
- Downregulation: ↓ receptor number (e.g., opioids, β-agonists).
- Desensitization: ↓ receptor affinity/coupling (e.g., GPCRs).
- Post-Receptor Changes: Altered intracellular signaling.
- Physiological (Counter-regulatory) Adaptation: Body initiates opposing effects.
- E.g., Na⁺/water retention with diuretics.
- Receptor Changes:
⭐ Pharmacokinetic (dispositional) tolerance, often due to CYP450 enzyme induction by drugs like barbiturates or rifampicin, leads to reduced drug efficacy due to lower plasma concentrations.
Mechanisms of Tachyphylaxis - The Flash Fade‑Out
Tachyphylaxis signifies a rapid, acute decrease in drug responsiveness after repeated administration. Key mechanisms include:
- Depletion of Mediators: Exhaustion of endogenous substances (e.g., norepinephrine by tyramine, ephedrine).
- Receptor Alterations:
- Phosphorylation: Alters receptor sensitivity.
- Internalization/Downregulation: Receptors removed from the cell surface.
- Conformational Changes: Receptor structure modified, impairing drug interaction.
⭐ Tachyphylaxis to indirectly acting sympathomimetics (e.g., ephedrine, tyramine) is commonly due to depletion of vesicular norepinephrine stores.
Clinical Examples & Management - When Drugs Lose Punch
| Drug Class/Example | Phenomenon (Tolerance/Tachyphylaxis) | Key Clinical Implication/Management |
|---|---|---|
| Organic Nitrates (e.g., Nitroglycerin) | Tachyphylaxis (Rapid) | Angina; nitrate-free interval (8-12 hours) needed. |
| Opioids (e.g., Morphine) | Tolerance (Pharmacodynamic) | Chronic pain; dose escalation; opioid rotation. |
| Beta-2 Agonists (e.g., Salbutamol) | Tolerance (Receptor downregulation) | Asthma; ↓ effect with frequent use; step-up therapy (e.g., add ICS). |
| Benzodiazepines (e.g., Diazepam) | Tolerance (Pharmacodynamic) | Anxiety/insomnia; ↓ efficacy with long-term use; taper dose. |
| Indirect Sympathomimetics (e.g., Ephedrine) | Tachyphylaxis (NT depletion) | Decongestants; rebound congestion (rhinitis medicamentosa); effect ↓ rapidly. |
High‑Yield Points - ⚡ Biggest Takeaways
- Tolerance: Gradual ↓ drug responsiveness with chronic use, requiring dose escalation.
- Tachyphylaxis: Rapidly diminishing response to a drug after acute, repeated dosing.
- Pharmacodynamic tolerance: Often involves receptor downregulation or desensitization.
- Pharmacokinetic tolerance: Typically due to ↑ drug metabolism (e.g., enzyme induction).
- Cross-tolerance: Tolerance to one drug extends to pharmacologically similar drugs (e.g., opioids).
- Nitrates exhibit tolerance to antianginal effects and tachyphylaxis to headache.
- Tachyphylaxis is common with indirectly acting sympathomimetics (e.g., ephedrine).
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