Answer

Yes, it is safe to take NyQuil and Tamiflu (oseltamivir) together. There are no known interactions between the two medications.

Tamiflu actually has very few known drug interactions. The prescribing information for the drug lists only one, the live flu vaccine:

"The concurrent use of TAMIFLU with live attenuated influenza vaccine (LAIV) intranasal has not been evaluated. However, because of the potential for TAMIFLU to inhibit replication of live vaccine virus and possibly reduce the efficacy of LAIV, avoid administration of LAIV within 2 weeks before or 48 hours after TAMIFLU administration, unless medically indicated."

The prescribing information does list another drug, probenecid, as causing Tamiflu concentrations to increase if taken at the same time. Nevertheless, if both are used together, no dosage adjustments of either are necessary:

"Coadministration of probenecid results in an approximate two‐fold increase in exposure to oseltamivir carboxylate [Tamiflu] due to a decrease in active anionic tubular secretion in the kidney. However, due to the safety margin of oseltamivir carboxylate, no dose adjustments are required when coadministering with probenecid."

Outside of what the prescribing information for Tamiflu lists, there have been a small number of studies that suggest a minor interaction with the anticoagulant warfarin, but reports have only been anecdotal and more research is needed.

Going back to NyQuil, it is important to note that it contains multiple active ingredients. Therefore, when exploring drug interactions, we need to take them all into account.

NyQuil contains:

  • Acetaminophen
  • Dextromethorphan
  • Doxylamine
  • Phenylephrine (NyQuil Severe only)

None of the ingredients in NyQuil affect Tamiflu absorption or effectiveness and are completely safe to take together.


Summary

Yes, it is safe to take NyQuil and Tamiflu together. There is no known drug interaction between the two.


References
  • Elsevier ClinicalKey: Tamiflu(Accessed 1/9/19)
  • Tamiflu Prescribing Information (Accessed 1/9/19)
  • Review of potential drug interaction between Oseltamivir and Warfarin and why it is important for emergency medicine physicians. PubMed (Accessed 1/9/19)