"Use it" means "administer epinephrine now".
I've been there. When my child has said, "My mouth feels itchy", or "I'm getting a hive on my lip," I've grabbed the bottle of Benadryl in one hand and the epinephrine in the other and watched closely. Who wants to stick a needle in a child and call an ambulance unless it's absolutely necessary?
According to Dr. Oppenheimer, use of epinephrine early in the anaphylaxis state may diminish the effect of late-stage anaphylaxis. He said that anaphylaxis is under-diagnosed and under-treated and he made it clear that the first line of defense is epinephrine, not an antihistamine, when an anaphylactic reaction is starting.
He shared that the triggers for anaphylaxis are Food (33% of the time), Unknown (25%), Insect (19%), Medications (14%), Other (9%, includes cat dander, latex, cleaning agents, etc.).
For me, Dr. Oppenheimer's talk was another good reminder. One quarter of the time, an anaphylactic reaction has an unknown cause. An antihistamine is not effective at halting anaphylaxis. While no one wants to use an epinephrine injector, it is our best defense against anaphylaxis.
It is comforting to be able to reach an on-call doc in a moment of uncertainty and panic. Call your doctor if you have the time. If not, talk to your doctor now, so you have a plan in place if an anaphylactic reaction occurs.