Additionally, in a multi-centre study across nine study centres in eight countries, researchers assessed the association of different forms of contraceptives including, combined oral contraceptives (non-iron-containing), depot injection (medroxyprogesterone acetate), subdermal implant (Norplant®), copper IUD, stainless steel ring IUD, and non-users of contraceptives on Hb and serum ferritin concentration in 2507 women (18–40 years) with anemia (Hb concentration < 120 g/L) over 12 months [48]. Here, GSTM1 is linked to anemia (phenotype).