Lupron and Antagon are injectable medications that are used to prevent premature release of LH hormone during a stimulation cycle for IUI or IVF. These two medications work through different mechanisms to prevent the LH surge. Lupron usually requires at least 7 days to effectively prevent an LH surge, whereas Antagon works within hours. This difference explains why the drug protocols that employ these two medications are so different. Premature ovulation during an IUI cycle can be dealt with by simply adjusting the timing of the IUI, so these medications are primarily used in patients undergoing IVF rather than IUI. For most patients undergoing treatment with IUI these medications are not needed unless a patient repeatedly experiences a premature LH surge during the treatment cycle. In such cases, these medications can allow for a more optimal stimulation and larger follicle sizes.