L-DOPA is a naturally occurring chemical which is a precursor to the neurotransmitter dopamine. Unlike dopamine, L-Dopa can cross the blood-brain barrier. This makes it useful for treating neurological diseases caused by a lack of dopamine in the brain. In its pharmaceutical preparation, it is used to treat Parkinson's disease, manganese poisoning and encephalitis lethargica as well as certain types of dystonia.
Because L-Dopa will also form dopamine outside the brain, it usually causes severe side effects to the nervous system unless it is combined with a chemical that inhibits dopamine production outside the brain.
Other side effects of L-Dopa are low blood pressure, Arrhythmia, nausea, bleeding in the digestive tract, hair loss, disorientation, anxiety, increased sexual desire, vivid dreams, insomnia, hallucinations, and narcolepsy.
Although patients can often have great improvement when started on L-Dopa, most patients develop a resistance to the drug with the symptoms returning once a maximum safe dose has been reached. In addition, when patients on L-Dopa stop taking the drug, they can often have a very sudden onset of their symptoms. It can also deplete seratonin unless seratonin precursors are given at the same time.