Tyrosine is a neurotransmitter precursor that raises plasma neurotransmitter levels (especially dopamine and norepinephrine), but it has little to no effect on mood in healthy people. It's also required for the synthesis of estrogen and testosterone. Because it is a precursor for both estrogen and testosterone, increased intake through food or supplements may have effects on hormone balance. Also, high doses of tyrosine can lead to skin reactions, headaches, and heartburn. In contrast, phenylalanine is not a precursor for any hormones. Instead, it functions as a substrate for producing dopamine, norepinephrine, epinephrine, and serotonin. Phenylalanine is also needed to make melanin, which gives you color your hair and skin. Lack of this amino acid can cause pigment deficiency disorders such as albinism. Increased intake of phenylalanine can lead to mental problems such as anxiety, depression, obsessive-compulsive disorder, and aggression. Too much of it can also cause hallucinations, confusion, tremors, and seizures. The two amino acids are closely related: they differ only by their first position hydrogen atom. Thus, when one is low energy, the other will be too. For example, phenylalanine cannot be converted into tyrosine, and vice versa.