If pills are taken properly, up to 99 percent effective. Many women take them for years with no side effects. But they do have downsides. Some studies have shown increases in the rates of certain types of cancer among women who have taken hormones. Many women experience a variety of side effects, including weight loss or gain, skin problems, cramping, menstrual problems, psychological issues, and other problems. Sometimes a woman experiencing side effects can switch brands to make the side effects go away. Sometimes, the side effects persist regardless of the brand or formulation. And sometimes, a person simply doesn't want to be on medication every day indefinitely.