(Reuters) - The U.S Drug Enforcement Administration (DEA) on Wednesday relaxed some restrictions on research evaluating cannabidiol, an extract of the marijuana plant, for medicinal use.

The modifications will ease some requirements imposed by the Controlled Substances Act on possession of cannabidiol (CBD) for a specific U.S. Food and Drug Administration (FDA)-approved research protocol, the DEA said.

So far, researchers who expanded the scope of their studies and required more CBD than initially approved had to request, in writing, for a modification to their DEA research registrations.

The modification had to be sanctioned by the FDA and DEA, potentially holding up research.

As part of the changes announced on Wednesday, a previously registered CBD clinical researcher who is granted a waiver can readily modify the protocol and continue research seamlessly.

The changes are effective immediately, the DEA said.

Drug policy reformers have been urging the U.S. government to ease restrictions on scientific research into marijuana's potential as medicine, saying the federal government's strict rules had prevented needed studies.

The Obama administration has allowed states to experiment with marijuana legalization, though the drug is classified as a dangerous narcotic with no medicinal value under federal law.

So far, about half of U.S. states allow marijuana for medical use.

A handful of companies are developing cannabis-derived drugs. Pioneering the effort is Britain's GW Pharmaceuticals, which is slated next year to deliver the results of four late-stage U.S. studies of its botanical pot-based epilepsy treatment.

INSYS Therapeutics Inc and Zynerba Pharmaceuticals Inc are working on much earlier stages of development with synthetic cannabis for a number of disorders.

(Reporting by Natalie Grover in Bengaluru; Editing by Sriraj Kalluvila)