The invention relates to 5 cyano 3 methyl pyridine 2 carboxylic acid [3 ((3R 6R) 5 amino 3 6 dimethyl 6 trifiuoromethyl 3 6 dihydro 2H (1 4)oxazin 3 yl) 4 fluoro phenyl)) amide in crystalline form to its preparation its medical use and to medicaments comprising said compound in crystalline form for use in the treatment of Alzheimer s disease.