Most of what I will repeat here comes from a blogger that goes by the moniker of Skeptical Raptor. In particular, his looking into the research. In his article, he cites two main sources (which unfortunately, some of us probably don't have access to depending on web settings):
• Huang CY, Choong MY, Li TS. Effectiveness of cupping therapy for low back pain: a systematic review. Acupunct Med. 2013 Sep;31(3):336-7. doi: 10.1136/acupmed-2013-010385. Epub 2013 Jul 25. Review. PubMed PMID: 23886511.
• Kim JI, Kim TH, Lee MS, Kang JW, Kim KH, Choi JY, Kang KW, Kim AR, Shin MS, Jung SY, Choi SM. Evaluation of wet-cupping therapy for persistent non-specific low back pain: a randomised, waiting-list controlled, open-label, parallel-group pilot trial. Trials. 2011 Jun 10;12:146. doi: 10.1186/1745-6215-12-146. PubMed PMID: 21663617; PubMed Central PMCID: PMC3141528.
The first critique is that traditional adherents to cupping talk about the body's qi... Whatever you may believe about cupping, let's dispense with the idea of qi. It's quackery and simply doesn't exist. Simon Singh and Edzard Ernst state in Trick or Treatment: The Undeniable Facts about Alternative Medicine
Scientists are still unable to find a shred of evidence to support the existence of meridians or Ch’i. The traditional principles of acupuncture are deeply flawed, as there is no evidence at all to demonstrate the existence of Ch’i or meridians. Acupuncture points and meridians are not a reality, but merely the product of an ancient Chinese philosophy.
According to Mark Crislip, cupping is “like most pseudo-medicine, it is an elaborate placebo with no real effects on real disease.”
Edzard Ernst, one of the leading scientists in debunking junk medicine, has stated:
In conclusion, this overview of SRs (systematic reviews) suggests that cupping may be effective for reducing pain. The evidence is insufficient for other indications. All SRs are based on primary studies with a high risk of bias. Therefore, considerable uncertainty remains about the therapeutic value of cupping.
Thank you to Jan Doggen for the Skpetoid link. That also gives some great gems regarding cupping. One telling thing is in the quote below (emphasis mine):
I searched and searched for any clinical trials of cupping as a treatment for any disease, but there simply aren't any. There is a large number of published articles in alternative medicine journals, nearly all from China, but none that come from any legitimate peer-reviewed journals. Part of the problem is that there is no specific condition that cupping is alleged to treat; even the Chinese articles are all over the map
Of course, there will be tons and tons of anecdotes as to why cupping works, and Brian does also address that (again, emphasis mine):
However, there's one very good reason that probably explains cupping's popularity in the modern world, despite its lack of any credible value. Modern cupping practitioners usually sell the service along with a massage, often both before and after the cupping procedure. Massage is extremely relaxing. It feels great and is a proven treatment for stress, anxiety, plus any number of muscular injuries and other pains.
Moreover, massage actually has a mechanism by which it provides relief. Popular research performed at McMaster University in Canada and published in 2012 analyzed samples of muscle tissue both before and after brutal, tissue-damaging exercise, and compared muscles that underwent therapeutic massage with muscles that did not. The analysis provided the mechanism and underscored what physical therapists and massage therapists have known for decades, that massage significantly improves the healing of muscle.
I think this quote from Skeptical Raptor sums it up best though:
Michael Phelps wins gold medals because he’s a freak athlete who trains at a level that is beyond my imagination. He doesn’t win those medals because he has a worthless and pseudoscientific procedure done to him. I don’t care what he says about cupping, it’s an anecdote at best.