1.   And where naturally occurring chemicals are extremely rare, the value of looking for them is offset by the high cost of such a task.

2.   Both Redux and Meridia affect a naturally occurring chemical in the brain called serotonin, though they do it in different ways.

3.   It works by temporarily blocking the transmission to the brain of a naturally occurring chemical called acetylcholine.

4.   They affect a naturally occurring chemical in the brain called serotonin, though they do it in different ways.

5.   These changes are caused by fluctuating levels of serotonin, a naturally occurring chemical in the body.