If you are currently being treated with any of the following medications, you should not use brewer's yeast without first talking to your healthcare provider.

Antidepressants, Monoamine Oxidase Inhibitors (MAOIs)
Brewer's yeast contains a significant amount of tyramine, a substance that should be avoided by individuals taking antidepressant medications known as monoamine oxidase inhibitors (MAOIs). Examples of MAOIs include phenelzine, tranylcypromine, pargyline, selegiline (also used for Parkinson's disease), and isocarboxazid. This interaction may lead to "hypertensive crisis," a rapid and severe increase in blood pressure that is characterized by nausea and vomiting, headache, and irregular heartbeat. This reaction may even result in a heart attack or stroke.

Narcotics for Pain
As with MAOI antidepressants, brewer's yeast may also lead to "hypertensive crisis" if taken with meperidine, a narcotic medication used to relieve intense pain.