As is often the case with many alternative healing practices, there are a bunch of studies on it, but they aren't really up to scientific snuff to definitively say how cupping actually works, or whether the improvements seen are from the therapy or just a placebo effect. Larger sample sizes, rigorous controlling, and standardized clinical settings are needed to give cupping the official seal of approval. For the most part, though, existing studies haven't found any negative side effects, so despite how it looks, cupping is pretty safe.
What the experts say
"It increases blood flow, can reduce muscle tension, and allows athletes or patients to feel improvements in their range of motion," physical therapist Marcus Williams, who uses cupping in practice at Ohio State University Medical Center, says. He adds that while cupping lacks clinical evidence, there are basically three theories to explain why athletes and other people seek it as a form of treatment: it increases blood flow to a local area; the tensile force of stretching the skin helps dull pain signals being sent to the brain; and hormones, like endorphins, are released to help heal the area.