Comprehensive Masterclass on Drug Interactions

Drug Interaction

occurs when the pharmacological action of a drug is altered by the concurrent administration of another drug, food, beverage, dietary supplement, or environmental chemical

Types of drug interactions

Pharmacokinetic (adme)
Pharmacodynamic (efficacy)
drug-food / beverage

drug-herb supplement

drug-lab test

Absorption Interactions

Gastric pH Alterations

Chelation and Polyvalent Cation Complexation

Gastric pH Alterations: Mechanism, Impact

Antacids (Al/Mg Hydroxide), H2 - Receptor Antagonists (Ranitidine, Famotidine), and Proton Pump Inhibitors (Omeprazole) elevate gastric pH.

Reduces dissolution and absorption of weak acid drugs requiring an acidic medium (e.g., Ketoconazole, Itraconazole, Iron / Ferrous Sulfate, Atazanavir).

Chelation and Polyvalent Cation Complexation: Mechanism, Classic Drug Pairs

Divalent (Ca2+, Mg2+, Fe2+, Zn2+) and Trivalent (Al3+) cations form insoluble, unabsorbable chelate complexes in the GI tract

Classic Drug Pairs: Tetracyclines (Tetracycline, Doxycycline) or Fluoroquinolones (Ciprofloxacin, Levofloxacin) + Milk / Antacids / Sucralfate / Iron Supplements.

_{^{Pharmacist Resolution: Separate administration times by at least 2 hours before or 4--6 hours after the cation.}}

GI Motility Changes

Metoclopramide: Accelerates gastric emptying → faster absorption rate of Paracetamol.

Anticholinergics / Opioids: Slow gastric motility → delayed drug absorption.

Adsorption in Gut

Cholestyramine / Activated Charcoal: Binds drugs like Digoxin, Warfarin, and Thyroxine in the gut lumen, preventing systemic absorption.

Distribution Interactions (Plasma Protein Binding Displacement)

Mechanism: Drugs that are highly bound to plasma proteins (> 90% bound to Albumin) can be displaced by a second drug with higher binding affinity.

Pharmacological Result: Increases the Unbound (Free / Active) Fraction of the drug, leading to transient toxicity.

Warfarin

displaced by Sulfonamides, Salicylates (Aspirin), or Fibrates → severe hemorrhage

Phenytoin

displaced by Valproic Acid or Salicylates → ataxia, nystagmus, CNS toxicity.

Bilirubin Displacement in Neonates

Sulfonamides displace bilirubin from albumin in newborns → Free bilirubin crosses the blood-brain barrier causing Kernicterus (encephalopathy)

CYP450 Inducers (Mnemonic: "PS PORCS" or "G-PASCS")

CYP Inducers increase hepatic enzyme synthesis, accelerating the breakdown of substrate drugs

CYP450 Inhibitors (Mnemonic: "SICKFACES.COM" or "G PACMAN")

CYP Inhibitors block active sites of metabolic enzymes, causing toxic accumulation of substrate drugs

"PS PORCS" or "G-PASCS"

P - Phenobarbital (Barbiturates)

S - St. John’s Wort (Hypericum perforatum)

P - Phenytoin

O - Oxcarbazepine

R - Rifampicin (Rifampin) - Most potent inducer!

C - Carbamazepine

S - Smoking (Polycyclic Aromatic Hydrocarbons) / Griseofulvin

_{^{Clinical Outcome: Concurrent use of Rifampicin or Phenytoin with Oral Contraceptives or Warfarin causes rapid breakdown of hormones/anticoagulant → Unintended Pregnancy or Thrombosis/Stroke.}}

"SICKFACES.COM" or "G PACMAN"

S - Sodium Valproate (Valproate)

I - Isoniazid (INH)

C - Cimetidine (H2 blocker)

K - Ketoconazole / Azole Antifungals (Itraconazole, Voriconazole)

F - Fluoxetine / Paroxetine (SSRIs)

A - Amiodarone

C - Clarithromycin / Erythromycin (Macrolides)

E - Ethynilestradiol

S - Sulfamethoxazole / Cotrimoxazole

C - Ciprofloxacin (Fluoroquinolones)

O - Omeprazole (PPI)

M - Metronidazole / Grapefruit Juice

Simvastatin + Clarithromycin: Mechanism, clinical impact/ outcome

Clarithromycin inhibits CYP3A4

Massive statin concentration → Rhabdomyolysis & Acute Renal Failure

Warfarin + Amiodarone: Mechanism, clinical impact/ outcome

Amiodarone inhibits CYP2C9

2-3x increase in Warfarin levels → Fatal Bleeding (Must reduce Warfarin dose by 50%)

Terfenadine / Astemizole + Ketoconazole: Mechanism, clinical impact/ outcome

Ketoconazole inhibits CYP3A4

Accumulation of parent antihistamines blocking I Kr potassium channels → Torsades de Pointes (Fatal Ventricular Arrhythmia)

Theophylline + Ciprofloxacin: Mechanism, clinical impact/ outcome

Ciprofloxacin inhibits CYP1A2

Elevated Theophylline levels → Seizures, Cardiac Arrhythmias

Excretion Interactions (Renal Mechanics)

Urinary pH Manipulation and Ion Trapping

Alkalinization of Urine: Administering Sodium Bicarbonate

Acidification of Urine: Administering Ammonium Chloride

Renal Tubular Secretion Competition: Probenecid + Penicillin, Methotrexate + NSAIDs, Lithium + NSAIDs / Thiazides

Alkalinization of Urine

Administering Sodium Bicarbonate (NaHCO3) alkalinizes urine → ionizes weak acid drugs (Aspirin / Salicylates, Methotrexate, Phenobarbital) → dramatically accelerates their elimination in overdose management.

Acidification of Urine

Administering Ammonium Chloride (NH4Cl) or Ascorbic Acid acidifies urine → ionizes weak base drugs (Amphetamines, PCP) → accelerates excretion.

Probenecid + Penicillin

Probenecid competitively inhibits Organic Anion Transporter (OAT) in renal tubules → blocks Penicillin secretion → prolongs Penicillin half-life and blood levels (Therapeutic exploitation).

Methotrexate + NSAIDs

NSAIDs inhibit renal OAT secretion and reduce renal blood flow via prostaglandin inhibition → Methotrexate toxicity (severe bone marrow suppression)

Lithium + NSAIDs / Thiazides

Thiazides cause sodium depletion → proximal tubule reabsorbs both Sodium AND Lithium → Lithium Toxicity (tremors, ataxia, seizures).

Pharmacodynamic (PD) Interactions

PD interactions occur when two drugs act on the same receptor, physiological system, or signal pathway, altering clinical response without changing drug concentrations.

Additive Effects ($1 + 1 = 2$)

_{^{Concomitant use of two drugs with similar pharmacological actions.}}

Example: Ethanol + Benzodiazepines (Diazepam) or First-Gen Antihistamines (Diphenhydramine) → additive Central Nervous System depression → profound sedation, coma, respiratory arrest.

Synergistic / Potentiating Effects (1 + 1 = 3 or 0 + 1 = 2)

_{^{The combined effect of two drugs is greater than the sum of their individual parts.}}

Sulfamethoxazole + Trimethoprim (Co-trimoxazole): Sulfamethoxazole inhibits Dihydropteroate Synthase; Trimethoprim inhibits Dihydrofolate Reductase. Sequential block in bacterial folic acid synthesis converts bacteriostatic agents into a potent bactericidal combination.

Penicillin + Gentamicin: Penicillin weakens the peptidoglycan cell wall, allowing Gentamicin (Aminoglycoside) to penetrate the cell and bind 30S ribosomes →powerful bactericidal synergy for enterococcal endocarditis.

Antagonistic Effects (1 + 1 = 0)

The action of one drug opposes or abolishes the action of another.

Pharmacological (receptor competitor)

Competitive: morphine (agonist) vs. naloxone (antagonist) at Mu receptors

Physiological (opposing pathways)

Epinephrine (broncodilation via beta-2) vs histamine (bornchoconstriction

Chemical (direct neutralization)

Protamine sulfate (+) neutralizing heparin (-)

Phenelzine, Tranylcypromine, Selegiline, Isocarboxazid

Mechanism: Monoamine Oxidase (MAO) enzymes in the gut wall and liver break down dietary Tyramine. MAO Inhibitors ‘blank’ block MAO-A

MAOI: trigger foods

Aged cheeses (Cheddar, Blue, Swiss), red wine, tap beer, fermented meats (pepperoni, salami), yeast extracts (Marmite, Bovril), fava beans.

Norepinephrine

Ingested tyramine escapes breakdown → enters systemic circulation → taken up into sympathetic nerve terminals → displaces massive amounts of stored ‘blank’

Tyramine interaction + trigger =

Hypertensive Crisis (systolic BP > 180 mmHg), severe occipital headache, intracranial hemorrhage, fatal stroke.

Grapefruit Juice Interaction

Mechanism: Grapefruit juice contains Furanocoumarins (e.g., Dihydroxybergamottin) that irreversibly inhibit intestinal wall CYP3A4 and P-glycoprotein (P-gp).

Grapefruit Juice Interaction: Impact, affected drugs

Impact: Increases oral bioavailability of CYP3A4 substrates (200-400% increase in AUC).

Affected Drugs: Statins (Simvastatin, Lovastatin, Atorvastatin), Calcium Channel Blockers (Amlodipine, Nifedipine, Felodipine), Cyclosporine, Sildenafil.

Vitamin K and Warfarin

Warfarin inhibits Vitamin K Epoxide Reductase (VKORC1).

drops INR below therapeutic target (2.0-3.0)

Vitamin K and warfarin impact: High consumption of Vitamin K-rich foods (kale, spinach, broccoli, green leafy vegetables) antagonizes Warfarin's anticoagulant effect → ‘blank’, increasing stroke risk.

The "4 G's" Herbal Bleeding Risk

Ginkgo Biloba, Garlic (Allium sativum), Ginseng, Ginger.

Warfarin, Aspirin, Clopidogrel, or NSAIDs

Interaction: Inhibit platelet aggregation when combined with ‘blank’ → severe spontaneous bleeding without necessarily altering INR.