Johnt, sorry for derailing your thread with my misunderstanding regarding the role of COMT inhibitors. Soccertese and Tupelo3, thanks for helping to clear up that misunderstanding.

John's initial question still stands: why isn't there a drug which just contains levodopa and entacapone?

As I said earlier, my understanding is as follows:
"If you had a drug which contained just levodopa and entacapone, you wouldn't be able to stop the DDC enzyme from converting a lot of the levodopa into dopamine, before the levodopa made it into the brain." Which means you would need to use a lot more levodopa, which means you would get more severe side effects (e.g. nausea).

Is my understanding correct? Are there any other reasons?