We give IM first IV second. I don't see how IV would be more 'dangerous' provided your giving at the correct concentration. If you're at the point of giving it IV the patient is in a bad state.
There's a difference in absorption and peak plasma levels. If we give 300 ug (0.3mg) IM, it has to diffuse through the connective tissue, into a capillary, into the venous circulation, etc. and it takes a while for it to get into the circulation. Case in point, slower onset and longer duration of pain relief with IM morphine.
If I give an IV dose of epinephrine, e.g. 100ug (0.1mg) of 1:10,000 IV, I get instant absorption, by definition, and I'm giving a huge pressor dose of epinephrine in one go (consider a normal epinephrine drip is 0.5 - 10 ug/min -- This is the dopamine equivalent of giving 200ug / kg in one minute, if you want to think about it another way).
So the coronaries, the heart, and the cerebral circulation are going to get exposed to much higher concentrations of epinephrine than they'll see if you give even a much larger amount IM.
This means you now have a hugely elevated risk for coronary vasospasm, cardiac arrhythmia, and a very sudden hypertension. The bonus is that the plasma concentrations will fall very rapidly, so this will probably be short-lived. But there's definitely associated risks.
* Anecdote. I had a near-death anaphylaxis patient, profoundly desaturated, SpO2 reading 68% (for whatever that's worth - we all know the accuracy is questionable at that level), with no palpable radials, ST @ 180 with multifocal PVCs, and intermittent hypoxic seizures. Horrible compliance. Two doses of 0.1 mg epinephrine 1:10,000 IVP later, and I've still got a train wreck, but I now have improved compliance, an SpO2 of 82%, radial pulses and a pressure of 190 / 120. [End result an ETT, another 0.8 mg of epinephrine IM, some benadryl, sedation, analgesia, MDI ventolin, etc.].
IV administration is there for the near death patient where there's minimal perfusion to the muscular tissues, and we can't wait for the effects of IM epinephrine to take place. I've heard of people giving very high doses of epinephrine IV, e.g. 0.3-0.5 mg doses to people in moderate distress, because "I have the IV, so why not?". This isn't smart, it's exposing the patient to a very unnecessary risk.
