Parkinson's patients sometimes complain that their symptoms are not due to their disease but to their medication. L-dopa alone can be converted to other structures, such as 3-O-methyldopa catalyzed by the enzyme catechol-O-methyl transferase (COMT) which can cross the BBB or reaches the brain. Indeed L-dopa is often prescribed along with COMT inhibitors, such as entacapone. But, COMT inhibitors can cause serious side effects, such as dyskinesia. It accelerates PD progression by inducing neuronal cell death through self-oxidation.

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