An effective scalar photon interaction emerges from two-body non-relat
ivistic spinless QED due to finite mass corrections. This has the inte
resting consequence that even in a non-relativistic theory, a hydrogen
ic atom in the metastable 2S(1/2) state can decay by emitting one sing
le photon. However, in a self-conjugate system like positronium, this
one photon decay mode via this effective scalar photon interaction is
totally suppressed. (C) 1997 Elsevier Science B.V.