Nurses and pharmacists (here, anyway) can train patients on how to use a syringe to administer medications. The only problem is that drawing a syringe during a reaction takes precision and fine motor skills, two things people having a reaction may not have by that point. Cheap, yes. Excellent option for first responder professionals, yes. Suitable for parents and caregivers of those with allergies, possibly. Suitable for self-injection, unlikely.