Nuclear stress tests involve significant radiation exposure equivalent to 400 chest x-rays. The benefit of information obtained from a nuclear stress test, when done properly, far outweighs any hazard from radiation exposure. However, pregnant women should not undergo a nuclear scan, and patients who undergo several procedures involving radiation exposure should discuss the issue with a doctor, so that the total dose does not exceed safety limits. During a typical nuclear stress test, you are exposed to 11 millisieverts of radiation.
This is about three times the dose of radiation you receive just for living your normal daily life for a year.