Pharmacology encompasses how drugs interact with the body. Pharmacokinetics describes what the body does to a drug through absorption, distribution, metabolism, and excretion (ADME). The half-life of a drug is the time for its blood concentration to decrease by half, with steady state reached after approximately 4–5 half-lives. First-pass metabolism occurs when orally administered drugs are partially metabolized by the liver before reaching systemic circulation, reducing bioavailability; drugs with high first-pass effect may require sublingual, transdermal, or intravenous routes. Pharmacodynamics describes what the drug does to the body, including receptor binding and dose-response relationships. The therapeutic index compares toxic and effective doses; narrow therapeutic index drugs (warfarin, lithium, digoxin, phenytoin) require careful monitoring, while wide therapeutic index drugs have a larger safety margin.
Many drug classes are commonly used in clinical practice. Antibiotics include bactericidal drugs that kill bacteria directly (penicillins, cephalosporins) and bacteriostatic drugs that inhibit bacterial growth and allow the immune system to clear the infection (tetracyclines, macrolides). Amoxicillin, a penicillin-class antibiotic, treats ear infections, strep throat, pneumonia, and urinary tract infections. NSAIDs (ibuprofen, naproxen, aspirin) inhibit cyclooxygenase enzymes to reduce pain, fever, and inflammation, with risks including gastrointestinal bleeding, kidney impairment, and cardiovascular events. Acetaminophen (paracetamol) reduces pain and fever without anti-inflammatory effect and is easier on the stomach but can cause liver damage in overdose. Opioids (morphine, oxycodone, fentanyl) bind to opioid receptors for severe pain but carry risks of addiction, respiratory depression, and overdose death; naloxone reverses overdose. Benzodiazepines (diazepam, lorazepam) enhance GABA activity for anxiety, insomnia, and seizures, with risks of dependence and dangerous interactions with alcohol or opioids. SSRIs (fluoxetine, sertraline) are first-line antidepressants that increase serotonin. Statins (atorvastatin, simvastatin) lower LDL cholesterol by inhibiting HMG-CoA reductase. ACE inhibitors and ARBs lower blood pressure by blocking the renin-angiotensin-aldosterone system. Beta-blockers slow heart rate and reduce blood pressure. Proton pump inhibitors (omeprazole, pantoprazole) suppress stomach acid production. Antihistamines block histamine receptors to relieve allergy symptoms. Corticosteroids reduce inflammation and suppress immunity. Metformin, the first-line oral medication for Type 2 diabetes, lowers blood sugar by reducing hepatic glucose production and improving insulin sensitivity.
Drug interactions occur when one drug alters the activity of another, potentially increasing toxicity or reducing effectiveness. The cytochrome P450 enzyme system in the liver, especially CYP3A4 and CYP2D6, metabolizes most drugs; inducers (rifampin, carbamazepine, St. John's wort) speed metabolism while inhibitors (ketoconazole, grapefruit) slow it. Pharmacogenomics studies how genetic variations affect drug responses, enabling personalized prescribing. Generic drugs contain the same active ingredient as brand-name drugs at lower cost and must meet the same regulatory standards. A black box warning is the FDA's strongest safety warning, indicating serious or life-threatening risks. Special considerations include polypharmacy (taking five or more medications simultaneously, common in the elderly and associated with drug interactions and falls), loading doses (higher initial doses to rapidly achieve therapeutic levels), and the placebo effect, in which patient belief produces real physiological changes even with inert treatment.