A metric is formal if all
products of harmonic forms are again harmonic. The existence of a formal metric
implies Sullivan formality of the manifold, and hence formal metrics can exist only in
the presence of a very restricted topology. We show that a warped product metric is
formal if and only if the warping function is constant and derive further
topological obstructions to the existence of formal metrics. In particular, we
determine the necessary and sufficient conditions for a Vaisman metric to be
formal.