A drug designed to treat lymphoma has been shown to relieve chronic fatigue syndrome (CFS) symptoms in two-thirds of patients across two small, clinical trials in Norway.

A larger follow-up study is now underway, but these early results provide important insight into the cause of the mysterious disease, which can make patients feel exhausted and foggy-brained, and leave them house-bound for years.

CFS - or Myalgic Encephalopathy, as it's officially known - affects 2.5 million Americans and 180,000 Australians, but scientists have struggled to pinpoint its cause or agree on the underlying mechanisms. As a result, many frustrated sufferers have been told that the problem is "all in their head" or to just get a good night's sleep. In fact, CFS was only officially classified as a disease in the US earlier this year

But the results of these early trials implicate antibodies in the condition, and suggest that CFS "may be a variant of an autoimmune disease," the authors from Haukeland University Hospital in Bergen, Norway, write in PLOS ONEAutoimmune diseases are brought about by a patient's immune system going awry and attacking their own tissues.

The trials involved a drug called rituximab, which is used to treat rheumatoid arthritis as well as the blood cancer lymphoma. It works on these conditions by wiping out nearly all of a patient's B-cells - the type of white blood cell that makes antibodies - effectively allowing the immune system to restart and stop attacking the body.

But the Norwegian researchers first got the idea to test it against CFS by accident back in 2004, after they gave the drug to a lymphoma patient who also happened to have CFS. Surprisingly, she experienced relief from both conditions, as Andy Coghlan reports over at New Scientist.

In 2011, they released results showing that 10 out of 15 CFS patients given the drug experienced relief, compared to none of the patients in the control group, who were given a placebo.

The team has now published a follow-up in PLOS ONE, which suggests that prolonged use of the drug can keep the condition under control for years. This latest study involved giving 29 people with CFS two initial doses of rituximab two weeks apart, and then following it up over the next year with boosters.

Eighteen of the patients reported experiencing relief, and even three years later many felt better.

"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," Øystein Fluge, one of the lead researchers, told New Scientist. 

One of the participants told Coghlan that she was "completely revitalised. Suddenly, I could be sociable again".

Of course, both of these studies are too small to say for sure whether the drug could be used to treat CFS, but a trial involving 150 participants is now planned.

And Fluge and his team think that the timing of the patient's recoveries and relapses provides an important clue that errant antibodies may be the mechanism behind CFS. As Coghlan reports:

"Relief started four to six months after the first dose of rituximab, approximately the time it would take for existing antibodies to be cleared from the body. Participants relapsed after about a year - roughly how long B-cells take to regrow and start making new antibodies."

They still need to find out which tissues are being targeted by these hypothetical errant antibodies, and what's causing CFS patient's immune systems to attack their own cells in the first place. But we're getting closer to answers, and after years of CFS sufferers being told that they're not really sick, that's pretty exciting.