I was asked on a course the other day how to be certain to give an epi pen. The answer is that anaphylaxis can be a rapidly developing condition and ultimately it can be fatal. My answer was that if in doubt give the epi pen. It is better for someone to have their medication and not need it than not have it when they need it. Even if it is not needed at that point in time the person will not come to any harm.
To find out more book onto an anaphylaxis update or first aid course.