Acetaminophen And Codeine and Homatropine: 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
Homatropine
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 pain medicine contains codeine, which can slow down your gut and make it harder to have a bowel movement or empty your bladder. Homatropine is an anticholinergic medicine that can cause the same kinds of slowing. When you take them together, those effects can add up.
In rare cases, the gut can slow down so much that it nearly stops, which is a serious problem called paralytic ileus. This is based more on how these drugs work than on lots of real-world reports, so try not to worry. Just watch for constipation, belly bloating, or trouble urinating, and let your pharmacist or doctor know. Your care team can manage this easily.
Effect: Additive reduction in GI motility and urinary tone from combining codeine (opioid) with homatropine (anticholinergic), theoretically increasing risk of severe constipation, urinary retention, and paralytic ileus.
- Mechanism: Additive effects on gastric/GI motility; opioid-induced plus anticholinergic-induced slowing.
- Direction: Additive PD interaction (increased adverse effect), not a PK/enzyme interaction.
- Evidence: Theoretical; severity major. Onset unspecified.
- Management: Monitor for reduced bowel sounds, abdominal distension, constipation, and urinary retention. Dose and duration may be individualized; consider prophylactic bowel regimen if clinically appropriate.
What happens
An increased risk of paralytic ileus
Interaction Deep Dive
Concomitant use of codeine with anticholinergic drugs may increase the risk of urinary retention and/or severe constipation, which may lead to paralytic ileus. Monitor patients for signs of urinary retention or reduced gastric motility when codeine is used concomitantly with anticholinergic drugs1.
Why it happens (mechanism)
Additive effects on gastric motility
How to manage this interaction
If both medicines have been prescribed, keep taking them as directed unless your care team tells you otherwise. Here is what typically helps:
- Your team may monitor you more closely for constipation, bloating, or trouble urinating, especially early on.
- The doses and how long you use them may be adjusted and individualized by your care team.
- Staying hydrated and staying active can help keep your gut moving.
Contact your pharmacist or doctor right away if you notice a swollen or painful belly, no bowel movements, nausea and vomiting, or difficulty passing urine.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Acetaminophen And Codeine and Homatropine together?
Codeine and homatropine can both slow your gut and bladder, and together they may raise the risk of severe constipation or a stalled bowel (paralytic ileus); take both as prescribed but watch for belly bloating, no bowel movements, or trouble urinating and tell your care team. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Acetaminophen And Codeine and Homatropine 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 Acetaminophen And Codeine and Homatropine interaction managed?
If both medicines have been prescribed, keep taking them as directed unless your care team tells you otherwise. Here is what typically helps: Your team may monitor you more closely for constipation, bloating, or trouble urinating, especially early on. The doses and how long you use them may be adjusted and individualized by your care team. Staying hydrated and staying active can help keep your gut… 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 Acetaminophen And Codeine or Homatropine 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: CARISOPRODOL, ASPIRIN, CODEINE PHOSPHATE oral tablets, carisoprodol, aspirin, codeine phosphate oral tablets. Ingenus Pharmaceuticals LLC (per DailyMed), Orlando, FL, 2021. DailyMed
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