The band's tour hit the skids last week (ends30Aug15) when they ended a concert in Utah early after Lemmy experienced breathing problems onstage.
They scrapped the next show in Denver, Colorado on Friday (28Aug15), blaming altitude sickness, and Lemmy then cut short another set in Austin, Texas on Tuesday (01Sep15).
The band axed the next stop on the tour in San Antonio, Texas on Wednesday (02Sep15), and now two more dates in Texas have also been cancelled.
In a statement posted online, Motorhead have thanked fans for their well wishes and assured them the tour will continue as soon as Lemmy is well enough.
The message states, "Regrettably Motorhead have had to cancel their Dallas and Houston appearances... Lemmy fully intends to resume touring duties as, and when, his doctors give him the all-clear. Stay tuned for further updates, and as always, thank you for your support, understanding and good wishes."
Lemmy, 69, has suffered numerous health problems in recent years and underwent heart surgery in 2013.