Can Milk Of Magnesia Cause Any Problem In Kidney Functioning?

In four years my eGFR has gone from 55 to 45. I do not have high blood pressure, diabetes, do not smoke. Nor do I know of any kidney disease in my family. I exercise faithfully and have a reasonably healthy diet. I do, however, have a longer than normal colon and was having a lot of problem with gas and constipation. My doctor recommended taking two tablespoon of milk of magnesia every night, which I have done for several years. Could this be something bothering my kidney function?