Fenfluramine and Acetaminophen And Codeine: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jun 30, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Acetaminophen And Codeine
Fenfluramine
No brand names on record
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
Your two medicines can each raise serotonin, a natural brain chemical. Fenfluramine boosts serotonin, and codeine (the opioid part of your acetaminophen-codeine pill) has mild serotonin activity too. Taking them together can add up and, rarely, lead to a condition called serotonin syndrome.
Watch for a fast heartbeat, high fever, sweating, shakiness or muscle twitching, restlessness, confusion, or an upset stomach. This concern is mainly theoretical, so please do not stop either medicine on your own. Your care team can safely manage this by watching you closely, and you should contact them right away if these symptoms appear.
Mechanism: additive serotonergic effects. Fenfluramine increases serotonin release/blocks reuptake; codeine has weak serotonergic activity, giving a pharmacodynamic (PD) additive interaction. The acetaminophen component is not serotonergic. Codeine is a prodrug activated via CYP2D6, but this interaction is PD, not PK, so activation status does not change direction.
- Direction: increased risk of serotonin syndrome
- Onset: unspecified
- Evidence: theoretical
- Severity: major
Management: if coadministered, monitor for autonomic instability, hyperthermia, hyperreflexia/myoclonus, agitation, and GI symptoms. Discontinue and provide supportive care if serotonin syndrome develops.
What happens
An increased risk of serotonin syndrome (hypertension, tachycardia, hyperthermia, myoclonus, mental status changes)
Interaction Deep Dive
Fenfluramine is a serotonergic drug and concomitant use with another serotonergic agent should be approached with caution. Serotonin syndrome may be life threatening and symptoms may include mental status changes (eg, agitation, hallucinations, coma), autonomic instability (eg, tachycardia, labile blood pressure, hyperthermia), neuromuscular aberrations (eg, hyperreflexia, incoordination), and/or gastrointestinal symptoms (eg, nausea, vomiting, diarrhea). If coadministration is required, monitor closely for signs and symptoms of serotonin syndrome. If serotonin syndrome develops, immediately discontinue therapy and initiate supportive symptomatic therapy1.
Why it happens (mechanism)
Additive serotonergic effects
How to manage this interaction
Keep taking both medicines exactly as prescribed unless your prescriber tells you otherwise. This risk is theoretical, but it is rated as serious, so your team may take extra care.
- Your care team may monitor you more closely when these drugs are used together.
- Learn the warning signs: fast heartbeat, high fever, sweating, muscle twitching or stiffness, agitation, confusion, or nausea/vomiting/diarrhea.
- Ask your pharmacist or doctor whether any of your other medicines also raise serotonin.
- Seek help right away if you notice these symptoms; treatment is stopping the drug and supportive care.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Fenfluramine and Acetaminophen And Codeine together?
Combining fenfluramine with acetaminophen-codeine can add up serotonin effects and, rarely, cause serotonin syndrome; keep taking both as prescribed but watch for fever, fast heartbeat, agitation, or muscle twitching and contact your care team if they occur. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Fenfluramine and Acetaminophen And Codeine interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How is the Fenfluramine and Acetaminophen And Codeine interaction managed?
Keep taking both medicines exactly as prescribed unless your prescriber tells you otherwise. This risk is theoretical, but it is rated as serious, so your team may take extra care. Your care team may monitor you more closely when these drugs are used together. Learn the warning signs: fast heartbeat, high fever, sweating, muscle twitching or stiffness, agitation, confusion, or nausea/vomiting/diar… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
Questions for your pharmacist
- Does my dose of Fenfluramine or Acetaminophen And Codeine need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (1)
- Product Information: FINTEPLA(R) oral solution, fenfluramine oral solution. Zogenix Inc (per manufacturer), Emeryville, CA, 2020. DailyMed
Keep reading about Acetaminophen And Codeine
Keep reading about Fenfluramine
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