In an early 1988 study published in the Journal of the Autonomic Nervous System, researchers injected atropine into the muscles of 20 healthy male adults. Eight of the participants had brown eyes, another eight had blue eyes, and four individuals had hazel eyes. Atropine is a drug used to treat symptoms of bradycardia, or a slowed heart rate of fewer than 50 beats per minute accompanied by unstable patient vital signs, according to 2023 updated research published in StatPearls.
Using two different devices to administer the drug, the researchers measured participant pupil dilation and heart rate within the first hour and a half of the intervention. The research findings showed that men with brown eyes experienced faster heart rate acceleration than those with hazel or blue eyes. The researchers theorized that melanin, such as that found within the iris, may modify the body’s response to atropine. This was reinforced in 2022 research published in the Journal of Controlled Release, in which the researchers point out that certain drugs — such as atropine, timolol, and levofloxacin — bind to melanin, thereby heightening the drug’s concentration and remaining in the body for longer periods of time.