While kefir can be a natural way to offer your dog probiotic supplements, it’s not entirely safe either. Although very low in lactose, milk kefir can still cause adverse reactions in some dogs as they have varying degrees of sensitivity. If your dog drinks too much kefir regularly, he may experience diarrhea, bloating, watery stools, and other digestive issues. Some store-bought kefir may also contain added sugar and artificial flavoring like xylitol which can be highly toxic to dogs even in small amounts. If your dog becomes ill or behaves unusually after drinking kefir, contact your vet immediately.