The present invention relates to crystalline Form A of 1-[(3R)-6, 8-difluoro-3, 4-dihydro-2H-1-benzopyran-3-yl]-1, 3-dihydro-5-[2-[(phenylmethyl)amino]ethyl]-2H-imidazole-2-thione and crystalline Form B of 1-[(3R)-6, 8-difluoro-3, 4-dihydro-2H-1-benzopyran-3-yl]-1, 3-dihydro-5-[2-[(phenylmethyl)amino]ethyl]-2H-imidazole-2-thione, processes for preparing the forms and their uses in medicine. The present invention also relates to the amorphous form of 1-[(3R)-6, 8-difluoro-3, 4-dihydro-2H-1-benzopyran-3-yl]-1, 3-dihydro-5-[2-[(phenylmethyl)amino]ethyl]-2H-imidazole-2-thione processes for preparing it and its uses in medicine.