Spectral electroretinography in thioridazine toxicity. In three patients with thioridazine toxicity, the electroretinogram (ERG) to red light was found to be below the average normal range. A significant increase in its amplitude appeared with cessation of therapy in two cases. A further deterioration of the ERG amplitude to all stimulus conditions (white, blue, and red lights) occurred when the dose of the medication was increased in the third patient. 