Can Nyquil or Benadryl cause a false positive for methadone?

Yes, the dextromethorphan in Nyquil and the diphenhydramine in Benadryl could both give false positives for Methadone

Hey Mmc,

The following lists some of the common medicine/conditions that can cause a false positive for opiates:

https://www.drugs.com/article/false-positive-drug-tests.html

You will see that Nyquil can cause a false positive for opiates as well as amphetamines.

Hope this helps,

Laurie