COMT and Parkinson disease: A potential link between polymorphisms of COMT gene and susceptibility to MLIC is proposed because of increased risk of MLIC in PD patients on higher cumulative levodopa dose (Scott et al., 2016) and also the probable relationship of high COMT activity genotype with faster levodopa catabolism and less stable serum and brain drug concentrations leading to higher optimal clinically levodopa daily dose (Bialecka et al., 2008; Lee, Lyoo, Ulamnen, Syvänen, & Rinne, 2001).