Butter can be an allergen of sorts for some people in the sense that they have an adverse reaction to it when it is ingested. A lot of people are what is called “lactose intolerantâ€, meaning their bodies are not able to properly tolerate dairy products like butter and milk as well as ice-cream or anything that is derived from milk/cream.
It is important to limit or completely cut out butter from your diet if you are lactose intolerant or have an allergy to it. Lots of people have food allergies and need to stay away from certain foods because they are not good for them in a number of ways.