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

Antidepressant Medications, Monoamine Oxidase Inhibitors (MAOIs)
Phenylalanine may cause a severe increase in blood pressure in people taking MAOIs (such as phenelzine, tranylcypromine, pargyline, and selegiline). This severe increase in blood pressure (also called "hypertensive crisis") can lead to a heart attack or stroke. For this reason, individuals taking MAOIs should avoid foods and supplements containing phenylalanine.

Baclofen
The absorption of baclofen, a medication used to relieve muscle spasms, may be reduced by phenylalanine. Therefore, it is best to avoid taking this medication with a meal, especially one that is high in protein content, or with phenylalanine supplements.

Levodopa
A few case reports suggest that phenylalanine may reduce the effectiveness of levodopa, a medication used to treat Parkinson's disease. Some researchers speculate that phenylalanine may interfere with the absorption of this medication. Therefore, phenylalanine should not be taken at the same time as levodopa.