Esters refer to different types of compounds and chemicals that manufacturers use subtly change their products. Using esters in a synthetic steroid can help the steroid better bond to receptors. This helps the steroid remain in the body and continue producing effects for more time. Though esters can take different forms depending on the product, the manufacturers of steroids often use some combination of oils or fats. When originally used in veterinary offices, Tren Acetate contained a high level of natural oils. Thoroughly mixing the steroid with the oil was the first step associated with using it. While the newer products still contain some fats, users will find that these oils don’t lead to weight gain and that they no longer need to mix the liquid.