Hello from JustAnswer.
The thirst is a natural response to how this medicine works. The medicine prevents the kidneys from absorbing all the glucose that is filtered, so that there is a loss of glucose int he urine and less glucose in the blood stream. As more glucose gets into the urine, it will cause fluid content of the urine to increase, so that there is also an excess loss of fluid through the kidneys. As your body loses fluid, it will stimulate thirst, and you will need to drink more fluids to maintain balance.
It is worth noting that it is the increased fluid volume of the urine that is causing the thirst. It is not that drinking more fluids will make the kidneys work harder.
The mere fact that you are losing fluids in the urine, resulting in greater thirst and drinking more fluids is not a problem that requires contacting your doctor. However, if the loss of fluids is so bad that you are unable to keep up with the increased fluid demands and become dehydrated, that would be a problem, and would be a reason to contact your doctor. The best way to keep tract of fluid status is to check your weight on a daily basis. If your weight decreases by 5-10% of your baseline weight, then that would be the point at which you should contact your doctor. Also, if there are other signs of dehydration, such as lightheadedness, a low blood pressure, or a rapid heart beat, that would also be a reason to contact your doctor. In addition, if the thirst and demand for fluids is so bad that it is interfering with your life or job, so that the side effect is too bothersome for you, that would also be a reason to contact your doctor.
Over time, if your blood glucose and HgbA1c does not improve with the Invokana, it is also a reasonable argument that continuing the drug for no real improvement in blood sugar control is not worth the side effect. But that assessment will take some time to determine.
If I can provide any additional information, please let me know.