Its just injectable progesterone. I'd suggest DOUBLE dosing the week before and after your trip (ie keep Regumate-ing every day that you are there, PLUS give a shot the week before, of, and after your trip). They are NOT interchangable; one is synthetic, the other is not. They both do the same job, but do it slightly differently, enough so that I'd worry about just swapping them back and forth as available (which is not what you are doing, I know). Its also very expensive... a dose can cost you $20+ per shot. You will also run into dosage issues... there is no standard dose (unlike the 1ml/100lbs for Regumate) and every vet has their own opinion on what is needed. Some aim high, others low. Not only that, but some people (myself included) think there is a "too high" dose, that is a standard dose for other vets. Very confusing! I personally would not depend on the shot as the primary drug for very important foals or very difficult mares. Maybe in combination with Regumate, but not as complete replacement. My two cents!