Skip to Content

Drug interactions between dextromethorphan / guaifenesin / phenylephrine and Prozac

Results for the following 2 drugs:
Prozac (fluoxetine)

Interactions between your selected drugs


dextromethorphan ↔ fluoxetine

Applies to:dextromethorphan/guaifenesin/phenylephrine and Prozac (fluoxetine)

Consumer information for this interaction is not currently available.

GENERALLY AVOID: Coadministration with potent CYP450 2D6 inhibitors (e.g., quinidine, terbinafine) may significantly increase the plasma concentrations of dextromethorphan in patients who are extensive metabolizers of this isoenzyme (approximately 93% of Caucasians and more than 98% of Asians and individuals of African descent). The proposed mechanism is inhibition of the CYP450 2D6-mediated O-demethylation of dextromethorphan. Studies in humans have shown an increase in systemic exposure of dextromethorphan of up to 43-fold when given concurrently with quinidine. Increased plasma concentrations increase the risk of dextromethorphan-related adverse effects (e.g., agitation, confusion, tremor, insomnia, diarrhea, and respiratory depression) and serotonin syndrome. However, this interaction has also been used clinically, with dextromethorphan in combination with quinidine indicated by some authorities for the treatment of pseudobulbar affect. Data evaluating the impact of this interaction in patients who are poor metabolizers of CYP450 2D6 are limited; most studies include extensive metabolizers of this isoenzyme. It is expected that poor metabolizers would have elevated dextromethorphan levels without concurrent quinidine

MANAGEMENT: The combination of dextromethorphan with potent CYP450 2D6 inhibitors should be generally avoided. Some manufacturers consider the concomitant use of dextromethorphan and selective serotonin reuptake inhibitors contraindicated. If use is considered necessary, the patient should be monitored for signs of dextromethorphan adverse effects (e.g., agitation, confusion, tremor, insomnia, diarrhea, and respiratory depression) and serotonin syndrome, and advised to notify their health care professional if these adverse effects develop or worsen. Dose reduction of dextromethorphan may also be required.


  1. "Product Information. Nuedexta (dextromethorphan-quinidine)." Avanir Pharmaceuticals, Inc, Aliso Viejo, CA.
  2. Capon DA, Bochner F, Kerry N, Mikus G, Danz C, Somogyi AA "The influence of CYP2d6 polymorphism and quinidine on the disposition and antitussive effect of dextromethorphan in humans." Clin Pharmacol Ther 60 (1996): 295-307
  3. Cerner Multum, Inc. "Australian Product Information." O 0
View all 6 references

Drug Interaction Classification

The classifications below are a guideline only. The relevance of a particular drug interaction to a specific patient is difficult to determine using this tool alone given the large number of variables that may apply.

Major Highly clinically significant. Avoid combinations; the risk of the interaction outweighs the benefit.
Moderate Moderately clinically significant. Usually avoid combinations; use it only under special circumstances.
Minor Minimally clinically significant. Minimize risk; assess risk and consider an alternative drug, take steps to circumvent the interaction risk and/or institute a monitoring plan.

Do not stop taking any medications without consulting your healthcare provider.

Disclaimer: Every effort has been made to ensure that the information provided by Multum is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. Multum's information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill, knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate that the drug or combination is safe, effective, or appropriate for any given patient. Multum Information Services, Inc. does not assume any responsibility for any aspect of healthcare administered with the aid of information Multum provides. Copyright 2000-2017 Multum Information Services, Inc. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the drugs you are taking, check with your doctor, nurse, or pharmacist.