Cupping therapy is very common in traditional Chinese medicine.

Over the 18 years I've been in China, I've done hundreds of cupping sessions, most of which were at the Hospital of Traditional Chinese Medicine. The effect is almost instantaneous, but do not expect it to cure everything.

My experience and views concerning Cupping Therapy. 

Over the years, I had cupping therapy several hundred times. Usually after scraping therapy. (Routine treatments in traditional Chinese hospitals)

• The suction creates light burns on the skin that have a circular form.
• It does not have as strong an effect as acupuncture, but it may relieve some symptoms, such as flu.
• The spots (light burns) on the body are not equal. Some are red-purple, and others show almost nothing. It is not random and is not explained by the technique.

I believe Cupping Therapy diverts inflammation from other body parts to the skin, which has a large contact area. (The burns "suck out inflammation.")

• The main side-effect is sometimes an inflammation of the burned area.

Alixin, my (Chinese) wife, cups me when I feel unwell. The relief is noticeable. 

The article at the attached link describes the mechanism of cupping. Link: TCM: Cupping, Scraping, Ginger Therapy.