Medication adherence is one of the biggest challenges doctors face when trying to control chronic conditions like diabetes and heart disease, and now a new study suggests that when patients are switched to generic medications with a different color or shape, they’re more likely to stop taking the drug.

In the study published earlier this week in the journal JAMA Internal Medicine, researchers from Brigham and Women’s Hospital culled through a database of patients who stopped taking anti-seizure medications for epilepsy and found that those who had been switched to a generic medication that looked different from their previous prescription were more likely to have discontinued their treatment.

Color switches appeared to account for 1 out of 400 cases of non-adherence, which isn’t huge, but can add up over time considering how often people refill medications; seniors, who typically take nine medications that are each refilled at least four times a year, may have 36 opportunities each year to get switched to different looking pills.

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“We teach patients to know what their pills look like—to avoid medication errors—but we also need to teach them that these pills could change on month to month basis,” said study leader Dr. Aaron Kesselheim, an internist at the Brigham. That can be very confusing for patients to grasp, and some may simply stop taking any pills that look different than previous ones.

Some pharmacies place stickers on a prescription bottle to note that a pill has changed in appearance, but, in an editorial that accompanied the study, scientists at the US Food and Drug Administration called for generic pill manufacturers to consider the new study finding when determining how to design their products.