Chemo drug still works with water

I'm taking a course of chemotherapy pills and drinking 2.7 litres of water a day. The pills are Xeloda (capecitabine). Would water dilute their effectiveness?

Capecitabine is a type of chemotherapy meant for advanced cancers of the breast and colon. Its claim to fame is that it comes in pill form, rather than the more traditional and less convenient IV drip.

Like pretty much all chemotherapy, it's a heavy-duty medicine with the potential for serious side effects. These include anaemia, a weakened immune system, diarrhoea, vomiting and rashes. Dosing must always be carefully considered by an oncologist (cancer specialist), especially in those who have kidney problems.

The manufacturer's published advice for capecitabine is to take the pills with water, and to drink approximately 2-3 litres of water a day.

As a general rule, the body doesn't like to be dehydrated. It's less healthy, less able to cope with toxins and less able to excrete wastes when it's dehydrated. This is why most people on chemotherapy have their liver and kidney function checked regularly. The liver detoxifies the blood, while the kidney filters out wastes. If they're not doing their job, you can get sick or even die.

A kidney requires a lot of blood flow to stay healthy. If a person gets critically dehydrated, the kidneys will fail. That's part of the reason why vomiting and diarrhoea can be so important to control in chemotherapy patients. Each can dehydrate you so quickly that your kidneys shut down. Staying well hydrated is one key to staying healthy.

Regarding the dilution of medicines in the body, consider that more than half our body's weight is made up of water. For a 70kg person, that amounts to about 40 litres of water.

Drinking a little bit more than usual would have a trivial effect on the overall concentration of a drug. Furthermore, capecitabine is taken up by actively dividing tissues (like cancers) where it works its way into their DNA and damages it, forcing the cells to die.

The tiny changes in concentration we're talking about wouldn't have any effect on the uptake of chemo into these cells. And the opposite scenario, dehydration, certainly could have a negative effect on one's health.

Perhaps more than any reader question to date, I'd remind you that my answers are for your education, to help you and other readers better understand the workings of the body.

If you want specific medical recommendations on your particular situation there is only one real resource: consult your own personal doctor. Thank you for the interesting question, and best wishes for good health.

Gary Payinda MD, is an emergency medicine consultant in Whangarei.

If you have a science or health topic question you'd like addressed, email drpayinda@gmail.com

(This column provides general information and is not a substitute for the advice of your doctor.)