People with red hair sometimes say that anesthesia doesn’t work as well on them. A common version of the story is a dental filling where the numbing shot didn’t quite take, and the dentist had to give a second one. It’s easy to dismiss this as coincidence. But researchers have taken the idea seriously for over two decades, and they’ve found a plausible biological reason it could be true.

That reason is a gene called MC1R , short for melanocortin-1 receptor. Nearly everyone with natural red hair carries variants of this gene that keep it from working normally. Red hair is uncommon worldwide , found in roughly 1% to 2% of people. It is much more common in parts of northern and western Europe, particularly Scotland and Ireland.

What The ‘Redhead Gene’ Normally Does

MC1R is best known for its job in pigment cells. When the receptor works as it should, it signals those cells to make eumelanin, the dark pigment behind brown and black hair and behind a suntan.

When the receptor is weakened or switched off, the cells make a different pigment instead. This one, called pheomelanin, is reddish-yellow, and it’s what gives red hair its color. It also explains why many redheads have fair skin that burns rather than tans. In most cases, a person needs to inherit the variant from both parents to end up with red hair.

If that were the whole story, MC1R would be a gene about appearance and nothing more. What researchers have learned is that it’s connected to a much larger hormone system in the body.

How The ‘Redhead Pigment’ Gene Connects To Pain

MC1R responds to hormones that the body makes from a shared starting molecule called proopiomelanocortin, or POMC. That molecule gets cut into several smaller hormones. One of them tells pigment cells what to do. Another is beta-endorphin, a natural painkiller that the body produces on its own.

Some of these hormones make pain feel stronger, while beta-endorphin makes it feel weaker. Normally the two stay in balance. When MC1R doesn’t work as expected, that balance can shift, and the change isn’t limited to the skin.

Studies in animals give the clearest picture of this so far. A 2021 study published in Science Advances looked at red-haired mice carrying the same kind of MC1R variant found in people, and showed that these mice could tolerate more pain than dark-haired mice. The difference didn’t come from the red pigment. It came from changes in the hormone signals, which seemed to favor the body’s own pain relief.

What It Could Mean For Anesthesia And Redheads

In people, a 2004 study published in Anesthesiology suggested that redheads needed somewhat more of certain inhaled anesthetics to be fully put under. A 2005 study , also published in Anesthesiology , found that local anesthetics, the kind used to numb a small area, were less effective in people with red hair. Researchers have also noticed that redheads tend to report more anxiety about going to the dentist. That makes sense if they’ve had more experiences of numbing that didn’t fully work.

The relationship isn’t always in the same direction, though. A 2005 study published in the Journal of Medical Genetics found that people with nonworking copies of the MC1R gene got a stronger effect from a morphine-based painkiller. If their natural pain-relief system is already set up differently, a stronger response to opioids would fit with that.

I want to be careful here, because the research doesn’t all agree. A 2024 review published in the Journal of Personalized Medicine found that results vary quite a bit from study to study. Some suggest that red-haired women are more sensitive to certain kinds of pain. Others suggest people with two copies of the variant have a higher pain tolerance.

The anesthesia findings are mixed as well. The early studies that reported a difference were small. A 2015 study published in the Canadian Journal of Anesthesia was much larger. It compared hundreds of self-identified redheads with similar patients during real surgeries and found no meaningful difference in how much anesthesia they needed.

One likely reason for the disagreement is that hair color is an imperfect way to sort people. Two people with similar red hair can carry different versions of the gene, and some people carry the variants without having obviously red hair at all. Different kinds of pain also travel along different nerve pathways, so a finding about one kind of pain may not apply to another.

For patients, the stakes are lower than they might sound. Anesthesiologists don’t hand out a fixed dose and walk away. They monitor each patient closely and adjust as they go, which is how differences between people are already handled. If you have red hair and you’ve had numbing wear off early or not take hold, it’s still worth mentioning to your doctor or dentist before a procedure.

Why This Matters Beyond Just Redheads

What I find most interesting about this is how much it changed the way scientists think about MC1R. For a long time it was treated as a pigment gene and little else. It now looks like part of a hormone network that affects several parts of the body, including how pain is felt.

It’s still an open question why red hair became common in some populations in the first place. One widely discussed idea is that lighter skin helped people in cloudy northern regions make enough vitamin D from limited sunlight, and red hair was carried along with it. If that’s right, the anesthesia effect was never the point. It’s a side effect of a trait that was shaped by sunlight long before modern medicine existed.

Surprised that a hair-color gene could potentially shape how redheads feel pain? See how well you really know what’s going on inside your own body with this science-backed test: Human Anatomy IQ Test