Hey There, welcome back to Ask D’Mine, our weekly advice column hosted by Wil Dubois in New Mexico, a veteran type 1 and diabetes author with experience working as a clinical diabetes specialist. This week, a reader wants to know just how their diabetes tech may play into cancer treatments. Ugh.

{Got your own questions? Email us at AskDMine@diabetesmine.com}


Nancy, type 1 from South Carolina, writes: I am on the new hybrid loop Medtronic pump with CGM. I am starting chemo for HER2 positive breast cancer. The chemo meds are Doxorubicin and Cytoxan for round one; and Taxol and Herceptin for round two. My tumor has been removed. My question is: Will I be able to manage blood sugar, or will it spiral out of control??

Wil@Ask D’Mine answers: Oh man. As if diabetes doesn’t suck enough. Wow, diabetes and cancer. My heart goes out to you and your family, and I’m sending all possible positive energy and best wishes your way.

Of course, you already know more about breast cancer than I’ll ever know, but for the benefit of our other readers the HER2 positive types of breast cancer are fast movers. For those interested in how things work under the hood, HER2, whose formal name is human epidermal growth factor receptor 2, is a gene/protein combo that deals with the growth of healthy breast cells. When mixed with cancer, this has the unfortunate effect of supersizing the growth of the cancer cells, which are already in the fast lane when it comes to cell growth in the first place. Sadly, that means HER2 breast cancers grow faster than other breast cancers, are more likely to spread to other parts of the body, and are more likely to come back in the future.


So why am I depressing you by reminding you of this information today? Because everyone—including you—needs to understand that fast-moving cancers need to be treated more aggressively and with stronger meds, a fact that’s going to matter to us on the blood sugar control front.

There is some good news here, however: We now have designer drugs specifically engineered to go after HER2 breast cancers. The Herceptin in your round two is an example of one of these new wonder drugs. Apparently it’s designed to block that growth signal from the HER2 proteins. Yay! But will it affect your blood sugar?

Actually, it might. But not in the way you’d expect. Elevated glucose isn’t a common side effect listed on the prescribing info sheet for the drug, which means that it’s unlikely the medication itself will raise your blood sugar. And although post market reports suggest that it might cause sugar trouble for some people, it’s a crazy-small number, less than one-half of one percent of folks who take Herceptin. Where you might have some trouble is indirectly from a common side effect of Herceptin: Diarrhea. 

Diarrhea, along with vomiting, which is another common chemo side effect, dump carbs from meals back out of your body off-schedule, before the carbs are absorbed into the bloodstream. That puts you at risk for low blood sugars from the insulin you took for food that exited in an untimely manner from one end of your body or the other. 


OK, while we’re on the subject of your specific meds, let’s quickly run through the rest of your list before we jump into the pump part of your question.

  • Doxorubicin: No official effect on blood sugar, but it’s another one of those puke and dash drugs. If works by blocking enzymes that help cancer cells grow and divide.
  • Cytoxan: No specific effect on blood sugar, but like all cancer drugs, the list of possible side effects is impressively long, and includes a high likelihood of the two side effects we’ve been talking about. It works against cancer in the “resting phase” of cell division. The med is a first generation cancer drug and is actually related to the World War I chemical warfare agent Mustard Gas. Remember that chemo works by nearly killing the patient to kill the cancer.
  • Taxol: It may or may not interest you to know that this med was developed from plant alkaloids, in this case the bark of the Pacific Yew tree. It works against cancer cell division by screwing with the cell’s microtubules. Again, there’s no specific glucose-raising side effect, but more vomiting and diarrhea.

So the good news here, if you can call it that, is that none of your chemo meds, on their own, should cause your blood sugar to go bonkers. But that said, there’s yet one more shadow. Apparently, steroids are often given along with chemo meds as a way to lessen nausea, which is a common side effect of all of the drugs you’ll be taking; and steroids are one of the worst things you can take when it comes to controlling blood sugar. Worse than cotton candy-topped macaroni and cheese with a side of ice cream drenched in fudge sauce.

Steroids will cause your blood sugar to surge like Old Faithful. Plus, I’ve read that cancer patients, when they aren’t throwing up, tend towards comfort foods. And who can blame them? But most comfort foods are notoriously high in carbs. So I’m sure glad you are on a CGM. That means, that whatever happens with your blood sugar, at least you’ll know it. It’ll be like CNN with those little news briefs that scroll across the bottom of the screen all the time.

By the way, as a matter of course in cancer treatment, for all the reasons above, insulin often needs to be increased for insulin-using PWDs getting chemo. And, in fact, insulin often has to be started on PWDs getting chemo that previously didn’t need insulin.

Anyway, I think that being on a pump is going to be a big advantage for you here, especially this pump—if it can keep up with the changing environment in your body quickly enough. This particular pump has the capability of adapting to your body’s changing sugar environment by automatically increasing or decreasing the flow of insulin in response to the CGM readings. Even at night when you are sleeping. Will you have sweet control? I rather doubt it, but I think you’ll do better with it on your team than you’d do with a conventional pump, or not pump at all.

The only downside of this pump is that there’s not much that you can monkey with when it comes to the pump’s settings if the built-in algorithms prove not to be nimble enough to keep up with the changes in your body. In automatic mode, the only user-adjustable variables are the insulin-to-carb ratio and the duration of insulin action. If you find the chemo essentially makes you more insulin resistant, one possible workaround to jump-start the algorithm would be to use a more aggressive IC ratio and a shorter duration of action. That will make insulin flow in higher volumes for meals and make the micro boli that replace the basal rate trigger more frequently. Also, touch base with your original pump trainer and see if she (he?) has any other tricks that might help.

But let’s keep our eye on the end game here, which is survival. You’ve got an aggressive, life-threatening cancer. I don’t want to say, “Screw the diabetes,” exactly, but it might need to take a back seat for a while. Do the best you can, but short of going DKA, don’t sweat the high blood sugars too much.

Just beat the damn cancer. Then you can focus on fixing the diabetes. It’s willing to wait. 


This is not a medical advice column. We are PWDs freely and openly sharing the wisdom of our collected experiences — our been-there-done-that knowledge from the trenches. But we are not MDs, RNs, NPs, PAs, CDEs, or partridges in pear trees. Bottom line: we are only a small part of your total prescription. You still need the professional advice, treatment, and care of a licensed medical professional.