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.
Unlike levonorgestrel, and like mifepristone , ulipristal acetate is embryotoxic in animal studies.  Before taking the drug, a pregnancy must be excluded.  The EMA proposed to avoid any allusion to a possible use as an abortifacient in the package insert to avert off-label use .  It is unlikely that ulipristal acetate could effectively be used as an abortifacient, since it is used in much lower doses (30 mg) than the roughly equipotent mifepristone (600 mg), and since mifepristone has to be combined with a prostaglandin for the induction of abortion.  However, data on embryotoxicity in humans are very limited, and it is not clear what the risk for an abortion or for teratogenicity (birth defects) is. Of the 29 women studied who became pregnant despite taking ulipristal acetate, 16 had induced abortions, six had spontaneous abortions, six continued the pregnancies, and one was lost to follow-up .