A popular massage treatment is generating unsettling reports from clients who say they felt achy, exhausted or headachy afterward. Some social media users describe the experience as feeling under the weather or as though an illness is about to begin.

The reaction has picked up the name “massage flu” online, but health experts stress that the phrase is not an actual medical diagnosis. Instead, it describes temporary symptoms that some people report following a massage.

W. Zach Smith, a physical therapist in Seattle and co-founder of HIDEF Physical Therapy, said he has encountered the phenomenon through patients. “This phenomenon is something I have definitely heard of and had patients tell me,” Smith told Fox News Digital.

Smith said the basic idea is not new, even if its viral label appears to be more recent. “The idea of this has been around a long time but usually described as fatigue and soreness,” he said. “The flu part of the name I have heard more recently.”

One possible reason for soreness is the sustained pressure applied during a massage, particularly when a client is not accustomed to the treatment. Muscles and surrounding tissue can react when substantial pressure continues for an extended period.

“Any time you go in and apply a lot of pressure to a muscle for a long period of time, it can essentially bruise the tissue,” Smith said. He added that the body may experience a mild inflammatory response after a massage, which can also contribute to soreness.

Drew Voelsch, a chiropractor at Forza Chiropractic in Illinois, has heard patients use more than one colorful label for the experience. Along with “massage flu,” some have referred to the temporary reaction as an “adjustment hangover.”

“The symptoms are real,” Voelsch told Fox News Digital, pointing to soreness, fatigue and mild headaches that may persist for 24 to 48 hours. At the same time, he distinguished the symptoms from the catchy online terminology, saying, “The label is social media.”

The location of a massage may also matter when headaches develop. According to Smith, massage involving the neck, scalp or jaw can trigger a headache because those areas may be involved in tension headaches.

Both experts rejected a widely circulated social media claim that post massage symptoms happen because the treatment releases toxins from the body. Smith dismissed that explanation directly, saying, “This is a myth.” He added, “There is no basis or science behind this.”

Voelsch agreed that massage should not be credited with flushing toxins from the body. He noted that the liver and kidneys are responsible for the body’s waste removal processes.

For people trying to avoid feeling poorly afterward, Voelsch offered several simple suggestions. He recommended drinking water, eating something light before the massage and engaging in gentle movement after the treatment.

Mild soreness after a massage is generally not considered a cause for concern, according to Smith. However, pain that is new, unexplained or does not resolve may warrant medical attention.

“If you are having a new, unexplained pain that is persistent, that would be a good time to call your doctor,” Smith said. His guidance draws a clear distinction between expected temporary soreness and pain that continues without an explanation.

The discussion comes amid growing attention to wellness treatments such as lymphatic drainage and ice sculpting. Ainsley Earhardt explored their popularity with Nancy Vega of The Shape of You Spa in Summit, New Jersey, and internist Dr. Michael Aziz, including their use for fluid retention, bloating and thermogenesis.