  We investigate the relation between the annihilation of dark matter (DM)
particles into lepton pairs and into 2-body final states including one or two
photons. We parametrize the DM interactions with leptons in terms of contact
interactions, and calculate the loop-level annihilation into monochromatic
gamma rays, specifically computing the ratio of the DM annihilation cross
sections into two gamma rays versus lepton pairs. While the loop-level
processes are generically suppressed in comparison with the tree-level
annihilation into leptons, we find that some choices for the mediator spin and
coupling structure lead to large branching fractions into gamma-ray lines. This
result has implications for a dark matter contribution to the AMS-02 positron
excess. We also explore the possibility of mediators which are charged under a
dark symmetry and find that, for these loop-level processes, an effective field
theory description is accurate for DM masses up to about half the mediator
mass.
