Control structures in Forth cannot be used interpretively, only in a colon definition14. We do not like this limitation, but have not seen a satisfying way around it yet, although many schemes have been proposed.
To be precise, in Standard Forth the control-flow words have no interpretation semantics, and in Gforth the interpetation semantics of the control-flow words are not useful for interpretive control flow (see Interpretation and Compilation Semantics).