Top 10 Critical Drug Reaction Facts Every New Doctor Must Know

1. Drug Reactions Are Unpredictable and Can Occur at Any Time
   • Many doctors assume that if a patient has been taking a medication for a while without issues, they are safe. However, adverse drug reactions (ADRs) can develop even after prolonged use due to changes in metabolism, cumulative effects, or newly developed sensitivities.
   • Immune-mediated reactions like Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN) may appear days to weeks after starting a drug.
   • Always educate patients about delayed-onset reactions, particularly with antibiotics, NSAIDs, and anticonvulsants.

1. Not All Drug Reactions Are Allergic Reactions
   • Many patients (and even some doctors) confuse ADRs with allergies. A true drug allergy involves an immune system response (e.g., anaphylaxis from penicillin), whereas intolerance or idiosyncratic reactions are due to non-immune mechanisms.
   • Red man syndrome from vancomycin is not an allergy but a histamine release phenomenon.
   • Always obtain a thorough history before labeling a patient as "allergic" to a drug.
2. Polypharmacy Increases the Risk of Adverse Drug Reactions
   • The more medications a patient takes, the higher the risk of interactions and ADRs.
   • Elderly patients and those with chronic illnesses are at the highest risk due to altered pharmacokinetics and pharmacodynamics.
   • Always review medications, especially in hospitalized patients, to prevent drug-drug interactions.
3. Beware of Drug-Drug and Drug-Food Interactions
   • Common examples include:
     • Warfarin and green leafy vegetables (vitamin K antagonism)
     • Grapefruit juice inhibiting cytochrome P450 enzymes, affecting statins and calcium channel blockers
     • MAO inhibitors and tyramine-containing foods leading to hypertensive crisis
   • Always cross-check medications for potential interactions, especially in complex cases.
4. Drug Reactions Can Mimic Other Diseases
   • Certain ADRs present like common diseases, making diagnosis tricky.
     • Drug-induced lupus from hydralazine or procainamide can be mistaken for systemic lupus erythematosus (SLE).
     • Lithium toxicity can mimic stroke with slurred speech and ataxia.
     • Statin-induced myopathy may be confused with polymyalgia rheumatica.
   • A high index of suspicion is essential when evaluating new symptoms in a patient on medications.
5. Recognizing Life-Threatening Drug Reactions is Crucial
   • Every doctor should be able to recognize and manage:
     • Anaphylaxis: Rapid onset, wheezing, hypotension; requires immediate epinephrine.
     • serotonin Syndrome: Hyperreflexia, clonus, fever from SSRI overdose or drug interaction.
     • Neuroleptic Malignant Syndrome (NMS): Fever, rigidity, autonomic instability from antipsychotics.
     • Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Mucosal involvement, widespread rash leading to skin detachment.
   • Early recognition and withdrawal of the offending drug are key.
6. Genetic Variations Affect Drug Metabolism
   • Some patients metabolize drugs differently due to genetic polymorphisms in enzymes like CYP450 or N-acetyltransferase.
   • Clopidogrel requires CYP2C19 activation; poor metabolizers may not get full therapeutic benefits.
   • Asians are more prone to carbamazepine-induced SJS due to the HLA-B*1502 allele.
   • Pharmacogenetic testing is becoming more available and should be considered in specific cases.
7. Reporting Drug Reactions Improves Patient Safety
   • ADR reporting helps regulatory bodies like the FDA, EMA, or WHO monitor drug safety.
   • Many doctors fail to report reactions, assuming they are known side effects.
   • In hospitals, use internal reporting systems; outpatient physicians should report to national pharmacovigilance programs.
8. Always Consider Drug Reactions in Sudden Deterioration
   • If a previously stable patient suddenly worsens, always suspect an ADR.
   • Beta-blockers can precipitate acute decompensated heart failure in borderline cases.
   • Proton pump inhibitors (PPIs) can cause hypomagnesemia, leading to arrhythmias.
   • A thorough medication history is often the key to solving diagnostic dilemmas.
9. When in Doubt, Consult a Pharmacist or Toxicologist

• Pharmacists have extensive knowledge of drug interactions, metabolism, and ADRs.
• In cases of overdose, toxicologists or poison control centers provide essential guidance.
• Never hesitate to reach out to specialists when managing complex drug reactions.