The 69-year-old musician has been suffering from a variety of health issues in recent years, including heart surgery in 2013, a stomach bug, complications from diabetes, and problems with his legs, which cause him to walk with a cane.
He fell ill again during a show in Salt Lake City, Utah on Thursday and the concert was cancelled after only three or four songs as he could not continue.
The group blamed the altitude for his sickness, writing on Facebook.com, "The people are great, but the air is just too thin. The high altitude makes it difficult for breathing, and that's what happened with Lemmy tonight in Salt Lake City. He feels very bad to have cut the show short, but being that high up, he had some trouble breathing well. Lemmy appreciates everyone's concern. The fans always rally round!"
The rockers are due to play in Denver, Colorado on Friday night (28Aug15).