"I was completely revitalised," says Karen. "Suddenly, I could be sociable again. I would go to work, go home, eat dinner and feel restless."
Karen (not her real name) experienced this relief from chronic fatigue syndrome while taking a drug that is usually used to treat the blood cancer lymphoma and rheumatoid arthritis (see "Karen's experience", below).
She was one of 18 people with CFS who reported improvements after taking rituximab as part of a small trial in Bergen, Norway. The results could lead to new treatments for the condition, which can leave people exhausted and housebound.
Finding a cause for CFS has been difficult. Four years ago, claims that a mouse virus was to blame proved to be unfounded, and some have suggested the disease is psychosomatic.
The latest study implicates the immune system, at least in some cases. Rituximab wipes out most of the body's B-cells, which are the white blood cells that make antibodies.
Øystein Fluge and Olav Mella of the Haukeland University Hospital in Bergen noticed its effect on CFS symptoms in 2004, when they used the drug to treat lymphoma in a person who happened to also have CFS.
Several months later, the person's CFS symptoms had disappeared. A small, one-year trial in 2011 found that two-thirds of those who received rituximab experienced relief, compared with none of the control group.
The latest study, involving 29 people with CFS, shows that repeated rituximab infusions can keep symptoms at bay for years.
"Eleven of the 18 responders were still in remission three years after beginning the treatment, and some have now had no symptoms for five years," says Fluge. "Suddenly, their limbs started to work again and their hands were no longer cold or sweaty."
"I am very intrigued by the rituximab story," says Nancy Klimas, an authority on CFS at Nova Southeastern University in Fort Lauderdale, Florida. "It's particularly exciting when people seem to have experienced very long periods of remission, and even speak of recovery," she says.