The Collaborative International Dictionary of English v.0.48:

injectable \injectable\ adj.
   1. capable of being conveniently injected; -- used of drug
      preparations, such as sterile lyophilize medicine in a
      serum-capped vial. Opposite of uninjectable.
      [WordNet 1.5 +PJC]

   2. Requiring injection for administration; -- of drugs,
      especially those not active by oral administration.
      Opposite of orally active.
      [PJC]

WordNet (r) 3.0 (2006):

injectable
    adj 1: (used of drugs) capable of being injected [ant:
           uninjectable]