most of the studies on OCD used very high doses of inositol.

This substance is an insulin enhancing agent, and does cross the blood brain barrier, but not easily.
http://www.springerlink.com/content/n6652nl8u2228u56/

I think response to inositol is highly variable from person to person.
500mg/day is rather low. On the opposite side, I have heard of 15 grams a day in some studies.

We use it here, and found that it hits a ceiling after several weeks/months.
And then the response changes. That may just be our experience. But given the fact that blood brain barrier may not allow much through it might be that it does become saturated in the brain after time, and dose can be lowered then.

I think inositol is very good for anxiety. That is where it works for my husband and son. I used it for six months, and reached a plateau with it, so I no longer take it. My anxiety levels are much lower now.

I think it varies from person to person alot, depending on brain chemistry--which is different for each of us.