# Drug Interaction Cases: Bradycardia, Clonus, and Metabolites

New symptoms after a second prescription may reflect enzyme inhibition or metabolite accumulation; the interaction timeline is part of the diagnosis.

![Infographic: Drug Interaction Cases: Bradycardia, Clonus, and Metabolites](https://urinenephrology.org/visual-reference/images/psychotropic-cases.png?v=20261003c)

## Case 31: new bradycardia after paroxetine

Paroxetine inhibits CYP2D6 and can increase metoprolol exposure. Assess pulse/BP, symptoms, ECG, timing, and other causes of bradycardia. Syncope, hypotension, or conduction block requires urgent care; coordinate an interaction-aware regimen change once stabilized.

## Do not convert AUC into a prescribed dose

An observed exposure multiplier does not mean a patient is literally taking that multiple of the dose. Genotype, formulation, timing, and other clearance pathways matter. Use pharmacology to explain the interaction, then use the patient’s measured response to guide care.

## Case 32: fever plus clonus

Linezolid has monoamine-oxidase-inhibiting activity. New agitation, diaphoresis, hyperreflexia, clonus, and hyperthermia with serotonergic exposure suggests serotonin toxicity and needs urgent assessment. Stop implicated exposure through the treating team and manage airway, temperature, autonomic instability, and complications.

## Dialysis changes supportive treatment

The source case includes rhabdomyolysis, hyperkalemia, and acidosis in a dialysis patient. Anuria makes routine high-volume rhabdomyolysis fluid protocols unsafe. Assess perfusion/congestion and use indicated dialysis for electrolyte, acid–base, or volume problems while treating the toxic syndrome.

## Case 33: parent level is insufficient

Bupropion’s active metabolites can accumulate in renal impairment. New agitation, hypertension, or seizure needs a broader emergency evaluation and medication review even with an unremarkable parent level. Product labeling recommends considering lower dose/frequency and close adverse-effect monitoring.

## Build the prevention handoff

At a new antidepressant or antibiotic prescription, record all serotonergic drugs, beta blockers, kidney function, formulation, and dialysis status. Give a clear symptom/contact plan. Avoid treating a single small-study dosing interval as a universal dialysis prescription.

## Supporting evidence

- [Official clinical guidance](https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=61e9cbe3-8211-42f3-b68f-581f56651919)
- [Official clinical guidance](https://www.fda.gov/media/188607/download)
- [PubMed 38951961](https://pubmed.ncbi.nlm.nih.gov/38951961/)

## Source lessons

- [case31 paroxetine metoprolol cyp2d6](https://urinenephrology.org/2025_UDPA_Lectures_Live/cases/case31_paroxetine_metoprolol_cyp2d6.html)
- [case32 linezolid serotonin syndrome dialysis](https://urinenephrology.org/2025_UDPA_Lectures_Live/cases/case32_linezolid_serotonin_syndrome_dialysis.html)
- [case33 bupropion dialysis metabolite](https://urinenephrology.org/2025_UDPA_Lectures_Live/cases/case33_bupropion_dialysis_metabolite.html)

Read alongside the full lessons; the findings and decisions shown here require the stated clinical context.