- Patients must not be taking monoamine oxidase inhibitors, such as Clorgyline, Iproniazid, Isocarboxazid, Moclobemide, Nialamide, Pargyline, Phenelzine, Procarbazine, Rasagiline, Selegiline, Teloxantrone, or Tranylcypromine because of drug interactions with dextromethorphan