I'm having some trouble understanding your questions. Do you mean does radiation cause you to gain weight? If so, the answer to that one is no.
The main types of internal radiation therapy (called brachytherapy) are intracavitary radiation and interstitial radiation. Both of these methods involve using radioactive implants, such as pellets, seeds, ribbons, wires, needles, capsules, balloons, or tubes. During intracavitary radiation, the material is placed in a cavity (space) in the body, such as the uterus. With interstitial radiation, the implants are placed in or near the cancer, but not in a body cavity. They are surgically implanted, not swallowed.
What pill are you referring to? I did not have any medication to take in conjunction with radiation and I am not aware of any medications (other than perhaps some topical ointments) that are prescribed for radiation. Are your talking about an aromatase inhibitor for estrogen positive cancer? If so, that has nothing to do with radiation. Generally aromatase inhibitors are taken for 5 years, but some recent studies indicate there might be some additional benefit to taking them longer. The side effects vary by person and by the particular aromatase inhibitor prescribed.