As of 2015, while normal amounts of phenylalanine in foods are likely safe, too much phenylalanine in a pregnant woman's system creates a risk for birth defects, states WebMD. Phenylalanine can also cause high blood pressure, mental retardation or other serious conditions in people with phenylketonuria or similar disorders.
Individuals taking levodopa for Parkinson's disease should not use phenylalanine as the interactions make Parkinson's worse, according to WebMD. Phenylalanine also reacts with phenelzine or tranylcypromine, depression treatment medications, as well as certain antipsychotic drugs doctors prescribe for mental conditions. People with schizophrenia should use caution when taking phenylalanine as it can complicate a movement disorder called tardive dyskinesia.