(Bloomberg) -- Thousands of UK patients with long Covid are leaving the country to seek expensive unproven treatments such as “blood washing” abroad, according to a report.

Many travel to private clinics in Cyprus, Germany and Switzerland for apheresis -- a blood-filtering procedure -- and anti-clotting therapy, according to the investigation published Tuesday in the BMJ medical journal. One patient reported paying more than 50,000 euros ($50,185) for apheresis, hyperbaric oxygen therapy and an intravenous vitamin drip at a center in Cyprus and returning home with no improvement. 

Researchers are still puzzled as to the exact cause of long Covid, which can appear with vastly different effects in various groups of patients. The UK has almost 2 million long Covid sufferers whose symptoms range can be as mild as fatigue and anxiety or as severe as chest pains and breathing difficulties, according to the country’s Office for National Statistics. 

There’s no internationally accepted treatment pathway for long Covid, according to a statement from the journal. Experimental therapies are sometimes given within clinical trials that are designed to determine value and effectiveness. Clinics should be prepared to collect data that would answer these questions, doctors cited in the report said. 

“The treatment of patients with long Covid outside of a clinical trial is of concern,” said Fergus Gleeson, a professor of radiology at the University of Oxford who is studying lung abnormalities in long Covid. 

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