The warm stimulation of a hydrotherapy spa bathtub prompts the brain to release endorphins, the "happy hormone." Its effects are similar to morphine, but without side effects. People feel light, calm, and even experience increased happiness during hydrotherapy sessions, which is a result of the effects of endorphins.

For patients with heart disease, high blood pressure, or low blood pressure, the use of a hydrotherapy spa tub requires special caution. The temperature of a hydrotherapy spa tub is usually between 36°C and 40°C. Soaking in hot water can cause vasodilation, leading to fluctuations in blood pressure.