Some side effects with Comtan are potentially serious and should be reported immediately to your healthcare provider. Although most of these serious side effects are rare, a few may be common. Many of these side effects have occurred with carbidopa-levodopa products, which have been available for a long time, and are expected to also be a problem with Comtan, since it is always taken with carbidopa-levodopa and since Comtan increases the level of levodopa in the blood. These serious side effects of Comtan (plus carbidopa-levodopa) include but are not limited to: