General Principles of Pharmacology in Dentistry
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Direct answer
Pharmacology for the dentist rests on two questions: what the body does to the drug (pharmacokinetics — absorption, distribution, metabolism, excretion) and what the drug does to the body (pharmacodynamics — receptors, dose–response, interactions). Half-life sets the dosing interval, bioavailability and first-pass metabolism set the route, and steady state arrives after four to five half-lives. On top of this science sits a legal skill: writing a prescription that satisfies the Drugs and Cosmetics Rules, with Schedule H, H1 and X requirements applied correctly.
What you must remember
- First-order kinetics (a constant fraction cleared per unit time) governs most drugs you will prescribe; zero-order, capacity-limited kinetics is the exam exception — ethanol, aspirin at anti-inflammatory doses and phenytoin, where a small dose increase produces a disproportionate rise in plasma level.
- Steady state is reached in 4–5 half-lives; a loading dose shortcuts the wait and equals volume of distribution × target concentration ÷ bioavailability.
- Therapeutic index is TD50/ED50; the narrow-index drugs a dentist meets are warfarin, phenytoin, digoxin, lithium and levothyroxine — monitor them, never titrate casually.
- First-pass hepatic metabolism slashes oral bioavailability: glyceryl trinitrin, propranolol and lignocaine are the textbook examples, which is why GTN is given sublingually in angina.
- Only unbound drug is active; acidic drugs such as warfarin, about 99 per cent protein-bound, can be displaced by competitors, transiently raising free levels and bleeding risk.
- Enzyme inducers (rifampicin, carbamazepine, phenytoin) cause treatment failure of co-prescribed drugs — oral contraceptive failure is the classic viva answer; inhibitors (erythromycin, ketoconazole, cimetidine) cause toxicity.
- A prescription has four parts — superscription (Rx), inscription (drug and strength), subscription (quantity) and signatura (directions to the patient) — plus patient identity, date and the prescriber's registration number.
- Schedule H1 drugs, introduced in 2014 for certain antibiotics and anti-tubercular drugs, must be recorded in a separate register retained for three years — a specifically Indian requirement examiners love.
A worked interaction: the warfarin patient who needs a tooth out
Take a 58-year-old on warfarin after a valve replacement who telephones with severe pain from a fractured molar. First, classify the risk before choosing the drug: warfarin is narrow-index, highly protein-bound and cleared by CYP2C9, so any new prescription is an interaction question. Second, choose the analgesic on pharmacodynamic grounds — paracetamol first, because NSAIDs add antiplatelet action and gastric erosion on top of anticoagulation. Third, if infection needs cover, recall the antibiotics that inhibit warfarin's metabolism and raise the INR: metronidazole, erythromycin, fluoroquinolones and co-trimoxazole; amoxicillin is comparatively safe but still deserves an INR check within three to five days. Fourth, plan the surgery around pharmacokinetics: a morning slot, an INR confirmed in range, local haemostatics and a 5 per cent tranexamic acid mouthwash. Fifth, counsel on the warning signs — bruising, dark stools, gum oozing — because the patient usually detects trouble before the clinic does. Every step chained kinetics, dynamics and prescribing law together, which is exactly what a viva examiner wants to hear.
Where students slip
Two confusions dominate. The first is swapping definitions under pressure: in a viva, "what the body does to the drug" must come out as pharmacokinetics without a pause. The second is assuming dose and effect move together: doubling phenytoin in a zero-order drug can triple the level, a fact that separates a pass from a distinction. Indian papers also reward the legal layer generic summaries skip — the four parts of a prescription is a perennial two-mark question, and candidates who can state that Schedule H1 adds the three-year register requirement to Schedule H routinely outscore those who only know the drug lists.
Frequently asked questions
How many half-lives are needed to reach steady state?
Four to five — about 94 per cent at four and 97 per cent at five, which is why loading doses are used when therapeutic levels are needed quickly.
Which drugs follow zero-order kinetics at therapeutic or toxic doses?
Ethanol, aspirin at anti-inflammatory doses, phenytoin at higher doses and theophylline near toxicity — elimination saturates, so levels climb disproportionately with dose escalation.
What are the four parts of a prescription?
Superscription (Rx), inscription (drug and strength), subscription (quantity to dispense) and signatura (directions to the patient), supported by patient details, date and prescriber particulars.
What distinguishes Schedule H1 from Schedule H in India?
Schedule H1, added in 2014, covers certain antibiotics and anti-tubercular drugs and requires the pharmacist to record the sale in a separate register kept for three years.
Why is glyceryl trinitrin given sublingually rather than orally?
Extensive first-pass metabolism destroys most of an oral dose; the sublingual route bypasses the portal circulation and acts within one to three minutes.
What happens when rifampicin is co-prescribed with warfarin?
Rifampicin induces hepatic enzymes, accelerating warfarin metabolism, lowering the INR and risking therapeutic failure — the INR must be monitored and the dose adjusted.