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In humans, tyrosine helps the brain build neurotransmitters known as catecholamines, which include dopamine, norepinephrine and epinephrine. 
L Tyrosine Benefits and Effects
Improving Working Memory
Tyrosine supplements can help replenish neurotransmitters necessary for maintaining working memory.  Therefore, some students like to take L tyrosine for studying.
Assisting the Thyroid
In studies of rats with thyroid deficiencies, subjects given tyrosine supplements performed better on cognitive tasks. 
Boosting Dopamine Levels
Taking tyrosine may also increase dopamine production when needed; however, the effect is temporary since taking L tyrosine in high doses can slow neuronal firing. 
Improving Stress Resistance
Rats who were given tyrosine injections before being subjected to electric shock maintained consistent norepinephrine levels and demonstrated no resulting cognitive deficits. 
Managing Attention Deficit Disorders
In one study testing the link between L tyrosine and ADHD, eight of 12 patients saw improvements following two weeks of tyrosine supplementation. However, those patients developed a tyrosine tolerance after six just weeks, so more research is needed on L tyrosine and ADD. 
In several small scale studies, tyrosine decreased symptoms of depression better than a placebo.  A larger study yielded mixed results, but the researchers posited that tyrosine is likely most effective at treating depression stemming from dopamine and norepinephrine deficiencies. 
Treatment of Parkinson’s Disease
Since Parkinson’s affects dopamine-producing neurons, tyrosine could help prevent progression of the condition. 
In a study of humans, subjects given tyrosine injections performed better on a stop-signal task than a control group. 
How L Tyrosine Works
The human body naturally produces tyrosine as a precursor to dopamine, norepinephrine and epinephrine. Thus, taking L tyrosine nootropic supplements can help the brain replenish these neurotransmitters as they get used up. 
L Tyrosine Energy
Tyrosine may only improve physical or cognitive performance in circumstances where catecholamine levels are being depleted. 
Standard dosing has yet to be established, but online L tyrosine reviews suggest taking 500-2000mg prior to an acute stressor. For best results, tyrosine should be taken 30-60 minutes before exercising or performing a mentally demanding tasks. It can be taken 2-3 times per day.
Taking L tyrosine long term appears to be safe for adults, but it’s not recommended to take L tyrosine during pregnancy. You should also avoid mixing L tyrosine and alcohol.
Unwanted weight gain as well as stiffness in the shoulders and neck have also been reported. 
L Tyrosine Drug Interactions
While L tyrosine has shown promise for treating Parkinson’s disease, it can interfere with some Parkinson’s treatments.  Likewise, although some people take L tyrosine for thyroid hormone production, combining tyrosine supplements with thyroid hormone pills can actually lead to hyperthyroidism. 
Ask your doctor about L tyrosine’s interactions with other medications you’re currently taking.
Taking tyrosine along with the tryptophan supplement 5-HTP can help combat depression since 5-HTP boosts serotonin, another neurotransmitter that affects mood.