Side effects of 21-day short-acting birth control pills include gastrointestinal symptoms, irregular uterine bleeding, etc. Harmful effects include the ability to cause weight gain in women and changes in menstruation. 1. Side effects (1) Gastrointestinal symptoms: Taking 21-day short-acting birth control pills can cause gastrointestinal symptoms, such as nausea and vomiting, but these gastrointestinal symptoms usually do not last more than 24 hours. (2) Irregular uterine bleeding: 21-day short-acting birth control pills will change the hormone levels in the patient’s body, so it may lead to irregular uterine bleeding. 2. Harms (1) Weight gain: 21-day short-acting contraceptive pills contain estrogen, and increased estrogen levels can cause water and sodium retention in the body, so it can cause women to gain weight. (2) Menstrual changes: 21-day short-acting contraceptives may cause endometrial dysplasia and insufficient glandular secretion, leading to menorrhagia or amenorrhea. There are many other side effects and hazards of 21-day short-acting contraceptive pills, in addition to liver and kidney insufficiency and other patients are contraindicated, while suggesting that if you need to use the drug should be used under the guidance of a doctor, do not blindly use the drug.