In the courses we have said that dogs in particular may display a special response when ingesting arnica oil. Sometimes dogs may begin to cough as though they had something stuck in their throat. This typically occurs when the issue at stake is more emotional than physical.
In this very brief video you can hear the type of sound I referred to. This video was made during the last days of my bearded collie, Sasha, so on the basis of this response it would appear that the emotional issues were more important than the physical discomfort or pain he may have been feeling.