When my petite friend Suki and her strapping husband were battling a bacterial infection at the same time, both her doctor and his prescribed the same antibiotic. And both patients were given the same dosage, even though Suki weighs half as much as her husband. Neither doctor made a mistake—they followed the standard protocol. But a recent article in the medical journal The Lancet may prompt doctors to reconsider.
I checked in with the article’s lead author, Matthew Falagas, MD, MSc, DSc, director of the Alfa Institute of Biomedical Sciences in Athens, Greece, and an adjunct associate professor of medicine at Tufts University School of Medicine in Boston. He pointed out, “Let’s say we have a woman who is four feet eleven inches and weighs 123 pounds, and a man who stands six foot three inches and weighs 198 pounds. Both those people have a similar body mass index (BMI) within the normal range, but of course the man’s larger body has more blood, more tissue and bigger organs. When we give both of them the same dose of a drug, they do not wind up with the same concentration of the medication.”
It’s a question of dilution. Think of it this way—when you stir a teaspoon of salt into a big pot of soup, your broth still tastes too bland… but if you add that teaspoon of salt to a bowl of soup, it winds up way too salty. Yet, drug manufacturers routinely take that approach with antibiotics and the human body, testing their products on people of middling size and weight and basing dosing guidelines on whatever works best for the average person. As a result, people who are smaller and thinner than average (as many women are) may get more medication than they really need, which increases their risk for adverse side effects.
One-size-fits-all dosing also creates problems for larger patients, particularly those who are obese, since they may not get enough medication to wipe out their infections. This could explain why obese patients are more susceptible to postsurgical infections despite being given precautionary antibiotics and why some of those infections require multiple courses of treatment, Dr. Falagas said. What’s more, when the hardier germs survive the too-wimpy antimicrobial assault, that particular type of bug could be encouraged to mutate into something stronger—contributing to the growing problem of drug-resistant bacteria.
It would be nice if the solution were as mathematically simple as “one pill for the 100-pound person, three pills for the 300-pound person.” But that’s not the case. Certain antibiotics dissolve more thoroughly in water while others interact better with fat, so a body that has more adipose (fatty) tissue will absorb and metabolize the medicine differently than the average body does. In addition, obesity affects the function of the liver and kidneys, where most medications are processed. And because obesity contributes to many chronic health problems, heavier patients are likelier to be taking multiple medications to deal with those problems, so there’s an increased potential for drug interactions.
Unfortunately, Dr. Falagas said, as yet, there is no simple method for taking all these factors into account to determine the most appropriate antibiotic dosage. So, for now, I’m afraid that we have to fall back on the common disclaimer that “more research is needed.” But the goal is not an impossible one. In some fields of medicine (anesthesiology, oncology, pediatrics), protocols are already in place for factoring in body size when prescribing drugs—because in these areas, giving too much or too little can have tragic consequences.
In the meantime: Until more research is done, doctors and pharmacists don’t have access to specific calculations to use, so in most cases, the standard antibiotic dosage remains the wisest course. If you’re a fairly average size, the standard dosage should be OK. Otherwise, Dr. Falagas suggested that you ask your doctor, “Considering that I am smaller/larger than the average person, do you think the standard dosage is appropriate for me?” At the very least, this should prompt your clinician to keep a closer eye on you whenever you’re battling an infection.
