You’ve heard of visual hallucinations, and know about hearing things that aren’t there. You’ve probably even experienced tactile hallucinations – like feeling your phone vibrate in your pocket when there’s no phone call. But did you know that you can have nasal hallucinations too?

At io9 they describe the phenomenon, and just how bad it can be:

Unlike real smells, hallucinations of smells don’t go away. People can’t turn away from them or open a window to dispel them. Some don’t even have the ability to get used to the smells. One woman smelled dirt for a year. After her husband burned chili one night, that smell replaced the dirt. A few years later, after a trip to France, she noticed that the scent of lavender had followed her back, and nothing she smeared under her nose took the scent away. Everyday activities became frightening, because she was never able to control what smell would take over her nose.

At the New York Times, that same woman recalls discovering that she has phantosmia:

I finally get it. This assertive smell, my uninvited companion for almost two days, is inside my head, not out. Mortified, I think I must smell. Talking to friends, I cover my mouth with my hand. I brush my teeth more often, swish mouthwash compulsively. But my husband says I smell fine — no bad breath. I finally call my doctor.

And phantom smells can be a sign of more serious brain troubles, according to NBC’s Body Odd blog.

Brief episodes of phantom smells or phantosmia – smelling something that’s not there – can be triggered by temporal lobe seizures, epilepsy, or head trauma. Phantosmia is also associated with Alzheimer’s and occasionally with the onset of a migraine.

As weird as this seems, phantom smells aren’t actually that uncommon.

Smell disorders aren’t that rare. According to a 1994 survey, 2.7 million Americans have some type of olfactory problem, including anosmia (the inability to smell); hyposmia (a decreased ability to smell); parosmia (a distorted perception, instead of flowers, you smell rotten meat), and phantosmia.

Smell might not seem like your most important sense, but when it takes on a mind of its own, it can really stink.

More from Smithsonian.com:

What Does Home Smell Like?

What Does Space Smell Like?