Depends on the situation.
There's a big heroin problem in the city where I work. There has been times people have wrecked their vehicles while driving..one instance, a person had been shooting up, while driving, and the tourniquet was still on their arm, needle by them, etc. At this point, I obtain an IV because it is a trauma as well as a probable OD.
In somebody with borderline vitals, I typically do IM..slower onset, less likely to get punched in the face before my 5 minute transfer to hospital.
If someone is completely unstable, I will give IN first to get it in their system as quickly as possible. If they don't come around relatively quickly, I will follow with IV or IM.
If all vitals are stable and breathing adequately, I just provide supportive care and withhold Narcan and let the ER decide if they want to give it or let the party come around on their own. I typically try to avoid throw downs in the back of the truck.