Can I cut olanzapine 20 mg tabs in half?

In theory, yes. However, the center oxidizes and breaks down in a few days to a few weeks. This is why it has an enteric coating and not recommended for splitting or crushing. So while it may be ok to split if both halves are taken in the same day, splitting could cause the medication to break down if left exposed to the air for too long.

A pharmacist would probably say no (they want you to purchase a new script) and they (Zyprexa) do (or did) used to have a smooth white coating over yellow insides; I guess I know that because I must have tried. I don't recall any issues and they still worked the same. They are not an extended release tablet but they may be coated to prevent extended exposure to stomach acid. The FDA document does not say one way or another.