Pharmacology Cheat Sheet
Drug Logic and Safe Prescribing
Pharmacology general principles for NEET-PG: pharmacokinetics, pharmacodynamics, therapeutic drug monitoring targets, trial metrics, and safe prescribing.
MedNext Academy | 3 min read
Drug Logic and Safe Prescribing
Pharmacology general principles for NEET-PG: pharmacokinetics, pharmacodynamics, therapeutic drug monitoring targets, trial metrics, and safe prescribing.
General pharmacology principles covering pharmacokinetics, pharmacodynamics, drug development, adverse reactions, therapeutic drug monitoring, and rational prescribing.
High-yield lines
- Pharmacokinetics is what the body does to the drug (absorption, distribution, metabolism, excretion), while pharmacodynamics is what the drug does to the body.
- Intravenous administration has a bioavailability of 100% by definition, whereas oral drugs are reduced by incomplete absorption and first-pass hepatic metabolism.
- The highest level of evidence is a systematic review or meta-analysis of randomised controlled trials; case reports and expert opinion rank lowest.
- Number needed to treat equals 1 divided by the absolute risk reduction, so a lower value means a more effective therapy.
- Therapeutic drug monitoring is reserved for narrow therapeutic index drugs such as digoxin, lithium, phenytoin, vancomycin, aminoglycosides, theophylline, carbamazepine, and valproate.
- The target range for digoxin is 0.5 to 2.0 ng/mL and for lithium is 0.6 to 1.2 mmol/L.
- Warfarin and heparin are monitored by their effect (INR and aPTT) rather than by measuring the drug concentration.
- Steady state is reached after about 4 to 5 half-lives, which is why a trough level checked too early reads falsely low.
- Tachyphylaxis is a rapid tolerance developing within minutes to hours, classically seen with nitrates and indirect sympathomimetics like ephedrine.
- CYP2D6 poor metabolisers get no analgesia from codeine, and CYP2C19 poor metabolisers respond poorly to clopidogrel.
- Low pseudocholinesterase activity causes prolonged suxamethonium paralysis, and G6PD deficiency causes haemolysis with primaquine, dapsone, and sulfonamides.
- Phase I trials test safety in 20 to 80 healthy volunteers, Phase II tests efficacy in patients, Phase III is large randomised trials, and Phase IV is post-marketing surveillance.
- Digoxin blood samples must be drawn at least 6 hours after the dose because earlier samples reflect the distribution phase and read falsely high.
- Idiosyncrasy is a genetically determined abnormal drug response, whereas allergy is immunologically mediated and dose-independent.
Mapped competency codes
- PH1.1
- PH1.2
- PH1.3
- PH1.4
- PH1.5
- PH1.6
- PH1.7
- PH1.8
- PH1.9
- PH1.10
- PH1.11
- PH1.12
Continue into the full chapter
This summary maps to PH1-general-pharmacology.
Frequently Asked Questions
Which drugs classically require therapeutic drug monitoring?
Narrow therapeutic index drugs: digoxin, lithium, phenytoin, carbamazepine, valproate, vancomycin, aminoglycosides, theophylline, and cyclosporin.
How many half-lives are needed to reach steady state?
Approximately 4 to 5 half-lives, unless a loading dose is used to achieve therapeutic levels faster.
How is number needed to treat calculated?
NNT equals 1 divided by the absolute risk reduction; a lower NNT indicates greater treatment effectiveness.
Why does codeine fail in some patients?
CYP2D6 poor metabolisers cannot convert codeine to morphine, so they get no analgesic effect.
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