Adverse drug reactions (ADRs) cause more hospital admissions, more morbidity, and more clinical regret than nearly any other category of medical error in neurology. Many are predictable from pharmacology — dose-dependent side effects of GABAergic ASMs, anticholinergic burden of older drugs, serotonin syndrome from polypharmacy. Others are idiosyncratic — hypersensitivity, severe cutaneous adverse reactions, drug-induced movement disorders. Beyond ADRs, neurology drugs require careful navigation of “special populations” — pregnancy, elderly, renal/hepatic impairment, pediatric patients — where standard dosing may be wrong or dangerous. This page covers the major ADR categories and the practical prescribing modifications for special populations.

Classification of Adverse Drug Reactions

Type A (Predictable, Dose-Related)

  • Augmented pharmacologic effect.
  • Predictable from known mechanism.
  • Dose-related: more drug → more effect.
  • Examples: sedation from benzodiazepines, ataxia from phenytoin overdose, bradycardia from beta-blockers, serotonin syndrome from excess serotonin.
  • Generally manageable by dose reduction.

Type B (Idiosyncratic, Non-Dose-Related)

  • Bizarre, unrelated to expected pharmacology.
  • Often immunologic or genetic basis.
  • Examples: lamotrigine SJS, valproate-induced pancreatitis, carbamazepine SJS in HLA-B*15:02 carriers, methotrexate hypersensitivity, fingolimod first-dose bradycardia.
  • Discontinuation needed; future avoidance.

Type C (Chronic Use)

  • Effects of long-term exposure.
  • Examples: tardive dyskinesia from long-term antipsychotics, osteoporosis from long-term steroids or enzyme-inducing ASMs.

Type D (Delayed)

  • Carcinogenicity, teratogenicity.
  • Examples: cyclophosphamide and bladder cancer, valproate and neural tube defects.

Type E (End of Treatment / Withdrawal)

  • Withdrawal syndromes.
  • Examples: benzodiazepine withdrawal (seizures, delirium), SSRI discontinuation syndrome, opioid withdrawal, levodopa neuroleptic malignant-like syndrome with sudden withdrawal.

Type F (Treatment Failure)

  • Inadequate effect, often due to interaction or genetic factor.
  • Examples: CYP2D6 poor metabolizer of codeine → inadequate analgesia.

Major ADR Categories in Neurology

CNS Depression and Sedation

  • Benzodiazepines, opioids, gabapentinoids, anticholinergics, older antihistamines, ASMs (especially older).
  • Cumulative with polypharmacy.
  • Risk: falls, MVCs, aspiration, respiratory failure.
  • Particularly dangerous in elderly.

Anticholinergic Burden

  • TCAs, paroxetine, oxybutynin, benztropine, scopolamine, antipsychotics (typical/atypical), antihistamines.
  • Effects: dry mouth, urinary retention, constipation, confusion, delirium, cognitive decline.
  • Anticholinergic burden scales used to quantify cumulative risk.
  • Particularly important in elderly and dementia.
  • Recent evidence links anticholinergic burden to dementia incidence.

Drug-Induced Movement Disorders

  • Akathisia: motor restlessness from D2 blockade (antipsychotics, metoclopramide, prochlorperazine).
  • Acute dystonia: often face/neck; benadryl IV/IM responds.
  • Parkinsonism: reversible with discontinuation; antipsychotics, metoclopramide, MOST commonly.
  • Tardive dyskinesia: late, often irreversible; long-term antipsychotic exposure; VMAT2 inhibitors (tetrabenazine, deutetrabenazine, valbenazine) for treatment.
  • Neuroleptic malignant syndrome: emergency.
  • Tremor: lithium, valproate, beta-agonists, amiodarone.
  • Chorea: oral contraceptives (rare), stimulants, dopaminergics in PD.

Serotonin Syndrome

  • Triad: mental status changes + autonomic hyperactivity + neuromuscular abnormalities.
  • Inducing drugs: SSRIs, SNRIs, MAOIs, tramadol, triptans (low risk), linezolid, lithium, ondansetron, methylene blue.
  • Often after dose increase or addition of second serotonergic drug.
  • Severe: rigidity, hyperthermia, autonomic crisis, death.
  • Treatment: cyproheptadine (serotonin antagonist), discontinuation, supportive.

Neuroleptic Malignant Syndrome (NMS)

  • Reaction to D2 antagonists or sudden dopaminergic withdrawal.
  • Hyperthermia, rigidity, autonomic instability, altered mental status, elevated CK.
  • Discontinue D2 antagonist; supportive care; dantrolene if severe.
  • Different from malignant hyperthermia (genetic; halogenated anesthetics).

Severe Cutaneous Adverse Reactions (SCAR)

  • Stevens-Johnson syndrome (SJS) / Toxic epidermal necrolysis (TEN): detachment of skin and mucous membranes; mortality 5-50%.
  • High-risk drugs: lamotrigine, carbamazepine, allopurinol, sulfonamides, abacavir, nevirapine.
  • HLA-B*15:02 + carbamazepine in Asians; HLA-B*57:01 + abacavir; HLA-B*58:01 + allopurinol.
  • DRESS (drug reaction with eosinophilia and systemic symptoms): organ involvement; weeks delayed.
  • Lamotrigine titration must be slow especially with valproate co-administration (potentiated by glucuronidation inhibition).

Hepatotoxicity

  • Valproate, methotrexate, statins, isoniazid, acetaminophen, many drugs.
  • Routine monitoring for high-risk drugs.
  • Valproate fulminant hepatitis: young children with metabolic disease at highest risk.

Hematologic Toxicity

  • Felbamate (aplastic anemia — rare but severe; restricted use).
  • Clozapine (agranulocytosis).
  • Carbamazepine (leukopenia, aplastic anemia rare).
  • Methotrexate.
  • Required monitoring with high-risk drugs.

Cognitive ADRs

  • Anticholinergics (cumulative).
  • Topiramate (“dopamax” effect — word-finding, slowed thinking).
  • Benzodiazepines (long-term cognitive effects, debated).
  • Opioids (acute and chronic).
  • Phenobarbital (especially in children).
  • Statins (often overstated; mild cognitive effects in some).

Mood ADRs

  • Antiepileptic drugs and suicidality: FDA black-box warning across all ASMs.
  • Levetiracetam: irritability, depression, behavioral effects.
  • Topiramate: depression.
  • Lacosamide: depression.
  • Interferon-β (MS): depression risk.
  • Steroids: mania, depression, anxiety, psychosis.
  • Anti-CGRP drugs (eptinezumab, fremanezumab, erenumab, atogepant, rimegepant): generally well-tolerated; rare mood effects.

Special Population: Pregnancy

Anti-Epileptic Drugs in Pregnancy

  • Valproate: HIGHEST teratogenic risk (neural tube defects, cardiac defects, autism); cognitive effects of in utero exposure significant. Avoid in women of reproductive age unless other options inadequate.
  • Topiramate: cleft lip, oral cleft risk; higher dose worse.
  • Levetiracetam: lowest teratogenic risk; preferred ASM.
  • Lamotrigine: low teratogenicity; preferred for childbearing-age women; PK changes (metabolism increase) require dose monitoring during pregnancy.
  • Carbamazepine: neural tube defects (slightly elevated).
  • Phenytoin: cleft palate; “fetal phenytoin syndrome” with chronic high doses.
  • Phenobarbital: cleft lip/palate; cognitive concerns.
  • Folic acid 4 mg daily for women on ASMs of childbearing age.

MS DMTs in Pregnancy

  • Most DMTs: contraindicated or to be avoided in pregnancy.
  • Glatiramer acetate: lowest risk; “no evidence of harm.”
  • Interferon-β: probably safe.
  • Natalizumab: rebound MS activity after discontinuation; some specialists continue through pregnancy with close monitoring.
  • Fingolimod, ozanimod, ponesimod: contraindicated.
  • Teriflunomide: known teratogen; washout protocol before pregnancy.
  • B-cell depleters (ocrelizumab, ofatumumab): may have effect on fetal B-cells; minimize during pregnancy.

Other Drugs

  • Triptans: probably safe; limited data.
  • Anti-CGRPs in pregnancy: limited data; generally avoid.
  • Levodopa for PD: probably safe.
  • Aspirin: safe.
  • Warfarin: contraindicated (use LMWH).
  • SSRIs: relative safety; weighed against untreated depression risk.
  • Lithium: cardiac defects (Ebstein); some specialists continue with monitoring.

Special Population: Lactation

  • Levetiracetam, lamotrigine: relatively safe.
  • Topiramate, valproate: usually safe; some pass into milk.
  • Phenobarbital: passes into milk; monitor infant.
  • Many DMTs: avoid breastfeeding.
  • Reference: LactMed database for individual drugs.

Special Population: Pediatrics

  • Different PK: greater volume of distribution (water), variable metabolism.
  • Different teratogenic risk windows.
  • Specific contraindications: valproate (especially in <2 yo if metabolic disease), aspirin (Reye), tetracyclines (<8 yo).
  • Behavioral and cognitive ADRs particularly important.
  • Many drugs lack pediatric data; off-label use common.
  • Antipsychotics: tardive dyskinesia risk over time; metabolic effects.

Special Population: Elderly

  • “Start low, go slow” — half typical adult starting dose; titrate cautiously.
  • Increased CNS sensitivity to benzodiazepines, opioids, anticholinergics.
  • Polypharmacy increases interaction risk.
  • Fall risk from CNS depression and orthostasis.
  • Renal function decline → adjust renally cleared drugs.
  • Beers Criteria: drugs to avoid in elderly.
  • STOPP/START criteria: similar framework.
  • Polypharmacy review at every visit.

Special Population: Renal Impairment

Drugs Requiring Dose Adjustment

  • Gabapentin, pregabalin: 100% renal clearance; substantial dose reduction.
  • Levetiracetam: 60% renal; reduce dose.
  • Topiramate: 70% renal.
  • Methotrexate: increased toxicity in CKD.
  • DOACs: dose by CrCl; rivaroxaban, apixaban acceptable in moderate CKD; many contraindicated in severe.
  • Acyclovir, ganciclovir: renally cleared; reduce dose.
  • Many antibiotics: dose reduction or contraindication.

Hemodialysis

  • Some drugs removed by dialysis (water-soluble, low Vd, low protein binding).
  • Phenobarbital: removed.
  • Lithium: removed.
  • Gabapentin: removed.
  • Phenytoin: NOT removed (protein-bound).
  • Lamotrigine: somewhat.

Special Population: Hepatic Impairment

  • Most ASMs hepatically metabolized; cirrhosis affects clearance.
  • Lamotrigine: reduce dose in moderate-severe cirrhosis.
  • Phenytoin: free fraction increases.
  • Many drugs require lower doses.
  • Acetaminophen: lower toxic dose threshold.

Polypharmacy and Deprescribing

  • Modern neurology patient often on 5-10+ medications.
  • Each medication adds interaction risk and ADR potential.
  • Deprescribing — systematic reduction of unnecessary medications — increasingly emphasized.
  • Particularly important: anticholinergics, benzodiazepines, gabapentinoids, opioids, statins of marginal benefit.
  • Use shared decision-making.

🔍 Did You Know?

Anticholinergic burden — the cumulative exposure to drugs with anticholinergic properties — has emerged in recent years as one of the most important and underappreciated risk factors for cognitive decline and dementia in older adults. Multiple large cohort studies have shown that chronic exposure to high-anticholinergic-burden medications is associated with a 25-50% increased risk of incident dementia over 10-15 year follow-up. The drugs that contribute include not only obvious culprits (oxybutynin, tolterodine for overactive bladder; older antihistamines like diphenhydramine; tricyclic antidepressants) but also commonly prescribed drugs that clinicians don’t always recognize as anticholinergic — paroxetine, benztropine, antipsychotics (typical and atypical), prochlorperazine, scopolamine patches, and many others. The Anticholinergic Cognitive Burden (ACB) Scale rates each drug 0-3; cumulative scores predict cognitive decline. The clinical implication is profound: deprescribing anticholinergic medications in older adults may slow cognitive decline, and avoiding cumulative anticholinergic exposure should be a priority in every dementia care plan. This effect is mechanistically plausible — basal forebrain cholinergic neurons are critical for memory and degenerate in AD; blocking their already-stressed system accelerates the process. For the neurologist, “what’s the anticholinergic burden of this patient’s medication list?” should be a routine question, particularly for patients with cognitive concerns. The lesson generalizes: medication review for ADR risk is as important as medication selection for therapy, and the elderly brain has a particularly low tolerance for cumulative pharmacologic insult.

Pitfalls and Pearls

  • Type A ADRs: dose-dependent; predictable; manageable by dose reduction.
  • Type B ADRs: idiosyncratic; often immune; discontinuation needed.
  • Anticholinergic burden: cumulative; associated with dementia; minimize in elderly.
  • Drug-induced parkinsonism: antipsychotics, metoclopramide — reversible with discontinuation.
  • Tardive dyskinesia: long-term antipsychotic exposure; often irreversible; VMAT2 inhibitors for treatment.
  • Serotonin syndrome: SSRI + tramadol/MAOI/triptan/linezolid; cyproheptadine + discontinuation.
  • NMS: D2 blockade + hyperthermia + rigidity; dantrolene + discontinuation.
  • SCAR (SJS/TEN/DRESS): lamotrigine, carbamazepine, allopurinol, sulfonamides; HLA pharmacogenomics.
  • Valproate in pregnancy: AVOID in women of reproductive age; teratogenic.
  • Levetiracetam, lamotrigine: preferred ASMs in pregnancy.
  • Folic acid 4 mg daily for women on ASMs of childbearing age.
  • Elderly: start low, go slow; review polypharmacy at every visit.
  • Beers Criteria, STOPP/START: frameworks for elderly prescribing.
  • Renal impairment: gabapentinoids, levetiracetam, topiramate need dose reduction.
  • Drug interaction databases: use Lexicomp/Epocrates for every new prescription.
  • Deprescribing: as important as prescribing in elderly patients.

References

  1. By the 2023 American Geriatrics Society Beers Criteria® Update Expert Panel. American Geriatrics Society 2023 updated AGS Beers Criteria® for potentially inappropriate medication use in older adults. J Am Geriatr Soc. 2023;71(7):2052-2081.
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  3. Coupland CAC, Hill T, Dening T, et al. Anticholinergic drug exposure and the risk of dementia. JAMA Intern Med. 2019;179(8):1084-1093.
  4. Tomson T, Battino D, Bonizzoni E, et al. Comparative risk of major congenital malformations with eight different antiepileptic drugs. Lancet Neurol. 2018;17(6):530-538.
  5. Boyer EW, Shannon M. The serotonin syndrome. N Engl J Med. 2005;352(11):1112-1120.
  6. Strawn JR, Keck PE, Caroff SN. Neuroleptic malignant syndrome. Am J Psychiatry. 2007;164(6):870-876.
  7. Reidenberg MM. Trends in adverse drug reactions. Clin Pharmacol Ther. 2018;103(4):582-585.