Some of the side effects of brachytherapy are similar to those seen with external beam radiation therapy, although they are usually less intense.

Examples of acute or immediate side effects include:

• Local swelling
• Local bruising
• Discharge in cases of vaginal or womb cancer.
• Semen may be discoloured and, in rare cases, may contain expelled pellets. Patients are therefore advised to use barrier contraception during sexual intercourse.
• Bleeding
• Pain and discomfort at the site of the implant
• General feeling of fatigue
• Brachytherapy used for uterine, cervical, vaginal or prostate cancer can lead to short-term urinary symptoms including urinary retention, pain on urination, incontinence and inability to urinate. Brachytherapy for these cancers can also lead to diarrhea, constipation and some rectal bleeding.

The side effects described usually resolve within a few days following the completion of treatment. However, severe bleeding, pain, the passage of blood clots and infection due to pellet implantation are side effects that all require immediate medical attention.

Long-term side effects include infertility and erectile dysfunction (in men treated with brachytherapy for prostate cancer). Those with permanent implants may be advised to avoid making close contact with people for the first few days after the implant when the radiation is most active. These individuals may also be told to restrict any daily contact with pregnant women or children to no more than a few minutes for the first few weeks or months. People with permanent implants should also be aware that the radiation in the brachytherapy seeds can set off airport radiation sensors.