NEW YORK (Reuters Health) - When it comes to lingering Lyme disease symptoms, intravenous antibiotics may not improve impaired thinking, two newly released studies suggest.

But one study found the treatment may help ease the debilitating fatigue experienced by many with post-Lyme syndrome, a condition in which symptoms continue even after initial treatment with antibiotics.

In an interview with Reuters Health, a co-author of that study cautioned that it was too soon to say that IV antibiotics should be prescribed to treat fatigue in patients who continue to experience symptoms after initial treatment for the tick-borne disease.

Even though the improvement in fatigue experienced by people receiving IV antibiotics "is provocative, this benefit needs to be weighed against the side effects of the treatment," said Dr. Leslie G. Hyman, a researcher in the department of preventive medicine at Stony Brook University Medical Center in Stony Brook, New York.

She also noted that "since fatigue is a non-specific symptom that can have a wide variety of causes, it is difficult to be sure that improvement in this symptom is due to persistent infection" with the Lyme disease bacteria.

Lyme disease is most common in the Northeast, parts of Wisconsin and Minnesota and northern California, although cases have been reported in other areas.

Besides developing a rash, a person bitten by an infected tick may develop flu-like symptoms. The illness is treatable with antibiotics, but if left untreated, people with the disease can develop serious complications, including arthritis and heart problems.

Some patients continue to have symptoms of Lyme disease after treatment with antibiotics, and researchers are divided over whether this represents continuing infection or a new illness, often called post-Lyme syndrome or chronic Lyme disease.

The two studies, which were published in the journal Neurology, were designed to test whether antibiotics beyond the initial course prescribed for Lyme disease could help knock back the long-term symptoms that a small percentage of people experience.

In the second study, researchers led by Dr. Richard F. Kaplan, a professor of psychiatry at the University of Connecticut School of Medicine in Farmington, followed 129 patients with a history of Lyme disease.

All patients also reported suffering one or more of the following symptoms: wide-spread musculoskeletal pain, impaired thinking, numbness, tingling, aching or burning sensations, as well as severe fatigue.

The patients were randomly assigned to receive either intravenous ceftriaxone for 30 days followed by 60 days of oral doxycycline or placebo intravenous treatment followed by 60 days of sugar pills.

When researchers compared the antibiotic-treated patients to the placebo-treated patients at 90 and 180 days, they found no difference in terms of cognitive function.

Researchers from Stony Brook also found little difference between antibiotic-treated and placebo-treated patients when it came to cognitive function.

In that study, researchers followed a total of 55 patients who had previously been treated for Lyme disease. At the outset, all 55 still suffered persistent, severe fatigue, according to Hyman. The patients were randomly assigned to receive either 28 days of intravenous ceftriaxone or an intravenous placebo.

When the researchers rechecked the patients six months after treatment, they found that 64 percent of people who had been treated with antibiotics felt less fatigued compared to 19 percent of those treated with placebo.

"This is an encouraging finding and further research should be done," Hyman said. "Since we don't know whether the benefit we observed for fatigue in our study is due to antibacterial actions or some other mechanism, other potential treatment approaches may go beyond antibiotics."

In an editorial accompanying the two articles, Dr. Israel Steiner suggests that neither could be expected to give a final answer on the subject of antibiotics and post-Lyme syndrome because there is no definitive test for the syndrome.

Without a clear test to make sure studies are including only patients with post-Lyme syndrome, "every attempt to address clinical questions in the realm of (post-Lyme syndrome) is doomed, almost by definition, to leave these questions unsettled," according to Steiner, who is at Hadassah University Hospital in Jerusalem.

Roche Laboratories provided the antibiotics and the placebo for the Stony Brook study.