Reprinted with permission of Jessical Jahiel, PhD, www.horse-sense.org
Mineral oil is precisely what you think it is: liquid petrolatum. It's not a laxative as such, but it's sometimes used by veterinarians when a horse presents with a case of impaction colic. The veterinarian will typically add a liter of mineral oil to a bucket of water and use a nasogastric tube and a pump to send the contents of the bucket directly into the horse's stomach.
Since mineral oil cannot be digested, it will travel through the stomach and intestines and eventually find its way out the other end of the horse - it's the veterinarian's (and owner's) hope that administering this mixture will do away with the impaction and bring the colic episode to an end. It's a crude method but can be quite effective: The water serves to soften the fecal material and the oil, as it passes through the intestines, can help shift the impaction and allow the fecal material to slip and slide through the intestines. An owner whose horse has impaction colic will eagerly await the sight of a pile of shiny, oily manure, because that will indicate that the blockage is not complete and that the oil is coming through and - one hopes - helping the horse pass manure more easily.
It's not a risk-free procedure and shouldn't be performed casually, for two reasons. First, passing a nasogastric tube requires careful attention. Veterinarians are always extremely careful to check the position of the far end of the tube BEFORE they begin sending the oil and water through it, because a very small error of position when the tube is inserted can result in the liquid going into the horse's lungs instead of its stomach...that error is very likely to be fatal. Second, if the colic is due to a twisted intestine rather than to an impaction, or if the impaction is very large, very hard, and the colic has been present for many hours, the situation may no longer be "horse with medical colic" but rather "horse with surgical colic." If surgery is on the agenda, it's NOT a good idea to administer mineral oil - it will only make the surgeon's work more difficult and increase the probability of post-surgical complications.
Vegetable oils in liquid form (and other forms of fat - rice bran, for example) won't serve to coat the manure in the horse's intestines and ease its passage because these oils will be digested, providing the horse with calories. As far as I know, the only danger of giving large quantities of such oils to horses would be the possibility of making the horses fat. If they were maintaining a suitable weight on their normal diet BEFORE the addition of large quantities of vegetable oils, then the addition of many calories from ANY source (that is, from fat or starch) will cause weight gain.
Since 1995, Jessica Jahile, award-winning author, clinician, and lecturer, has been answering questions from subscribers through her HORSE-SENSE Newsletter, a free Q&A newsletter dealing with all aspects of horses, their management, riding, and training. www.horse-sense.org