Thermotherapy should generally not be used during the first several days after an injury, because heat increases blood flow and may worsen swelling. For example, soaking a sprained ankle in warm water will increase pain and swelling and prolong the injury, according to the American College of Foot and Ankle Surgeons. Patients are advised to use cold therapy (cryotherapy) first to reduce inflammation before switching to thermotherapy. A physician can help a patient decide when to switch from cryotherapy to thermotherapy.

Heat therapy should not be used on tissue that has received radiation treatment or on tissue that is directly over a cancer site. People with poor sensation, such as some individuals with diabetes, are at increased risk of burns. In addition, pregnant women should not use any form of heat therapy that exposes their fetus to prolonged heat. This includes hot tubs.

Other potential risks associated with thermotherapy include:

• Hot packs. May increase core body temperature and blood flow. In some cases they may aggravate the inflammatory response. In addition, hot packs can cause burns. 
  
  
• Paraffin baths. Paraffin can burn the skin. This therapy should not be used by patients who have active bleeding or infection, neoplasm (abnormal growth of tissue such as a tumor), skin desensitization or vascular insufficiency (insufficient blood flow to a particular part of the body).
  
  
• Ultrasound therapy. May cause cavitation (gas bubble formation in tissue) or superheating of the periosteum (fibrous membrane covering the surface of bones) over bony surfaces. This therapy should not be used near the spine in patients who have had some types of spinal surgery, such as those that expose the spinal cord. It also should not be used over the eye, fluid-filled sacs or neoplasms. Pregnant women should not receive this treatment near their abdomen. Patients who cannot perceive pain or heat should not be treated with ultrasound thermotherapy.
  
  
• Diathermy. There is a risk of tissue burns associated with this therapy. Microwave or shortwave therapy should not be used by patients who are pregnant or who have a pacemaker or any other implants with electrical leads.
  

• Heat creams and ointments. Patients should avoid getting these ointments into their eyes or in contact with sensitive skin.  Patients should not combine other forms of thermotherapy with topical analgesics such as pain-relief ointments. Chemicals in these medications increase blood flow to the skin surface and may hinder the thermoregulatory response of heat redistribution. This can cause burns.