Thermotherapy is the application of heat to the body to relieve symptoms of acute or chronic pain. It is especially effective in treating pain related to muscle tension or spasm, but can also relieve other types of pain. Moist heat appears to be more effective in treating pain than dry heat because the moisture allows the heat to penetrate more deeply.

Many people use forms of thermotherapy at home to treat minor conditions, such as a hot pack applied to a stiff neck. Thermotherapy is also used in healthcare settings, such as physical therapy, manipulation therapy and occupational therapy. For example, a physical therapist may apply therapeutic ultrasound to a limb before exercise, a massage therapist may apply heated rocks to an aching back, and an occupational therapist may dip an arthritic hand in a warm paraffin bath before addressing fine-motor activities.

Thermotherapy produces several benefits that help relieve pain. Heat can increase blood flow to the skin. Application of heat produces vasodilation (widening of blood vessels), which increases oxygen and nutrient delivery to tissues. Heat relaxes muscles near the surface and makes them more elastic, decreasing stiffness of joints. Research has shown that superficial heat can also block internal pain receptors.

Heat also helps prepare the skin for the application of physical agents. Patients often find the application of moist heat to be soothing and relaxing.

Thermotherapy is often used combination with other therapies to relieve pain. In many cases, cryotherapy (cold therapy) is used to reduce inflammation before thermotherapy is applied to increase blood flow to muscles. Thermotherapy frequently incorporates moisture or water (hydrotherapy). Some forms of electrical therapy supply heat to injured tissues.

Thermotherapy generally should not be used during the first several days after an injury, because heat increases blood flow and may worsen swelling. Heat treatments are inappropriate for some individuals and conditions. For example, people who have impaired sensation (e.g., from neuropathy due to diabetes) are at risk of burns from overuse of thermotherapy. Individuals with poor sensation or other conditions of concern are advised to consult their healthcare provider before trying thermotherapy. Caution should be used not to overdo the therapy, which may burn the skin.