Acetaminophen And Codeine and Nelfinavir: 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
Nelfinavir
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.
Here's what's going on. Your pain medicine contains codeine, which your body turns into morphine to relieve pain. Nelfinavir (Viracept) can slow down one of the liver enzymes that handles codeine. That means more codeine and more morphine can build up in your body than expected.
When opioid levels climb, you can get very drowsy and your breathing can slow down, which can be dangerous. This is based mostly on how these drugs work rather than lots of real-world reports, but it's worth taking seriously. Please don't stop either medicine on your own. Your doctor or pharmacist can manage this safely by adjusting your dose and watching you more closely.
Mechanism: Nelfinavir inhibits CYP3A4, reducing the minor CYP3A4-mediated metabolism (N-demethylation) of codeine. This can increase codeine and its active metabolite morphine exposure, raising the risk of sedation and potentially fatal respiratory depression.
- Direction: Increased opioid effect (increased codeine/morphine concentrations).
- Evidence: Theoretical; magnitude not quantified.
- Onset: Unspecified.
- Management: If used together, monitor closely for respiratory depression and sedation; consider codeine dose reduction until stable effects are seen. On nelfinavir discontinuation, monitor for opioid withdrawal and consider increasing the codeine dose. Dose should be individualized by the care team.
What happens
Increased codeine and morphine exposure
Interaction Deep Dive
Taking codeine together with CYP3A4 inhibitors has led to elevated levels of codeine and morphine, potentially intensifying or extending adverse reactions and possibly producing fatal respiratory depression, especially when the inhibitor is introduced after the patient has reached a stable codeine dose. When such combined use is necessary, watch patients carefully for sedation and respiratory depression, and consider lowering the codeine dose until consistent drug effects are seen. Conversely, when a CYP3A4 inhibitor is being withdrawn, observe patients for indications of opioid withdrawal and consider raising the codeine dose until stable drug effects have been established12.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated codeine metabolism
How to manage this interaction
Keep taking both medicines as prescribed unless your care team tells you otherwise. This combination can be managed.
- Your care team may adjust and individualize your codeine dose and monitor you more closely, especially soon after starting these together.
- Watch for excess drowsiness or slow, shallow breathing, and get help right away if these occur.
- If nelfinavir is ever stopped, tell your team, since your codeine may need to be re-adjusted and they'll watch for withdrawal signs.
- Ask your pharmacist before adding any new sedating medicines or alcohol.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Acetaminophen And Codeine and Nelfinavir together?
Nelfinavir can raise codeine and morphine levels, increasing the risk of heavy sedation and dangerously slow breathing. Keep taking both as prescribed and let your care team adjust the codeine dose and monitor you. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Acetaminophen And Codeine and Nelfinavir 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 Nelfinavir interaction managed?
Keep taking both medicines as prescribed unless your care team tells you otherwise. This combination can be managed. Your care team may adjust and individualize your codeine dose and monitor you more closely, especially soon after starting these together. Watch for excess drowsiness or slow, shallow breathing, and get help right away if these occur. If nelfinavir is ever stopped, tell your team, s… 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 Nelfinavir 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 (2)
- Product Information: Codeine sulfate oral solution, codeine sulfate oral solution. Hikma Pharmaceuticals USA Inc(per FDA), Berkeley Heights, NJ, 2023. DailyMed
- Product Information: Codeine sulfate oral tablets, codeine sulfate oral tablets. Hikma Pharmaceuticals USA Inc. (per FDA), Berkeley Heights, NJ, 2023. DailyMed
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