Hi Doc. I've Been Taking Lithium Carbonate 900 Mg (2

Hi doc. I've been taking Lithium Carbonate 900 mg (2 x 450 morning & evening) very successfully for 29 years for bipolar disorder. I have been in the habit of breaking them in half to make them go down the hatch a little more easily, but I have just seen warnings on a couple of medical sites that this should not be done. Why is that, and what negative effects might I have been getting?