LONDON -- In a major study of medicinal marijuana, multiple sclerosis patients said using a pot pill made them feel better, but the improvements did not show up in a standard test.

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The British researchers said one explanation for that may be the test, which involves moving the patient's limbs. Researchers said it may not be sensitive enough to detect changes that are meaningful to patients.

The British study, published in this week's issue of The Lancet medical journal, involved 630 MS patients at 33 neurology and rehabilitation centres across the United Kingdom. Participants took either cannabis oil, a synthetic or a fake for 15 weeks.

About two-thirds of patients on the cannabis-based treatments said they felt their movement had improved, as did almost half of the patients taking a dummy pill.

"The results of this study present an interesting and complex picture of the value of cannabis-derived medicines for treating MS," said Dr. John Zajicek, one of the study's lead researchers. "The primary aim of the trial was to measure, as objectively as possible, the actual physical changes in limb spasticity in MS patients, and we found no evidence of this."

Many patients figured out which pill they were taking, which researchers said could have colored the results. Doctors said smoking marijuana is not an option because the smoke causes cancer.

"Science isn't always black and white, and this work underlines the importance of funding large-scale clinical trials of possible treatments so the evidence can be weighed up," said Colin Blakemore, chief executive of the U.K. Medical Research Council, which funded the study.