Metahumans are altered human beings that possess superhuman abilities. Humans can become metahumans by mutations (chemical or kryptonite exposure) or by birth. Those who acquire metahuman powers as a result of kryptonite exposure are sometimes referred to as "meteor freaks."
Alien characters whose abilities surpass those of humans are also sometimes classed as metahuman, though they are not actually human. 
Many of the metahumans Clark Kent has encountered are psychotic versions of their former selves who kill and injure innocent people with no remorse or reserve. The fact that they are still human means Clark has to inventively come up with non-lethal ways to defeat them. Even though, on fewer occasions, Clark has inadvertently killed metahumans to stop them, he still uses restraint while fighting them (the only exception being when he battles other threats such as the Zoners or Brainiac) . As a result, many dangerous metahumans are being held in Belle Reve. Most of them are teenagers, which could mean that children were more susceptible to the effects of meteor rocks. Unlike Clark, most do not fear others finding out about their powers. Many of them can have their powers controlled or subdued, and others' powers have spontaneously disappeared (such as Jordan Cross and Shelby), but there is no easily accessible or universal cure yet for kryptonite exposure.
In many cases, the circumstances of the meteor exposure appear to influence the types of powers developed. For example, Sean Kelvin nearly froze to death in a meteor-filled frozen lake and became a body-heat vampire, Tyler Randall came back from the dead due to meteor exposure and developed a touch of death, Sarah Conroy was asleep when exposed and developed dream-walking powers, etc.
In 2007, Dr. Curtis Knox claimed he could cure powers obtained by meteor rock infection, and the resulting psychosis, with brain surgery. While working with Lex Luthor, he cured Sasha Woodman and she was released from Belle Reve with a clean bill of health. However, his procedure was linked with the side effect of several years of memory loss.
There has been a significant decrease in the numbers of meteor freaks, as many were killed and captured by Lex's Level 33.1.
For a complete list of metahumans, see Category:Metahumans.
It has been shown that exposure to kryptonite can cause mutations in humans that cause them to develop a variety of superpowers. Below is an example of these so-called "meteor freaks" from each season. (See kryptonite effects on humans for more information.)
- Season One: Coach Walt Arnold was exposed to meteor rocks in his sauna. They gave him the power of to start fires with his mind. (Hothead) An elderly woman named Cassandra Carver at the Smallville Retirement Center could foresee the future of whoever she touched, she told Clark his destiny was to help people. (Hourglass)
- Season Two: Desirée Atkins was exposed to meteor rocks during the meteor shower while engaged in sex. They gave her the ability to influence men with super-pheromones. (Heat)
- Season Three: Jake Pollen had the ability to breath underwater. The nature of his exposure to the meteor rocks is not known. (Extinction)
- Season Three+Four: Alicia Baker had the ability to teleport herself except when protected by lead. The nature of her exposure is not known. (Obsession, Unsafe, and Pariah)
- Season Five: Buffy Sanders was bitten by kryptonite-infected rabid bats which gave her super strength but forced her to suck the blood of others to survive. (Thirst)
- Season Six: Duncan Allenmeyer was exposed to meteor rocks during an experimental treatment for his coma. This gave him the ability to project himself out of his body. (Reunion)
- Season Seven: The Weather Girls (Carly, Tempest Grace, and Tyler Crenshaw) were best friends who had elemental powers (heat, weather and cold, respectively). They used their powers to steal valuables. (Fierce)
There are definitely some meteor-infected people who are leading perfectly normal lives, using their abilities minimally. For example, in Freak, it is shown that LuthorCorp is kidnapping metahumans, doing tests on them and then releasing them with a blank memory about the night of their kidnapping. Tobias Rice was blinded by the second meteor shower and gained the ability to identify other meteor-infected people; he was unknowingly helping Lex find the meteor-infected. Unfortunately all the meteor-infected that were captured died, except for Chloe Sullivan.
Metahumans not infected by kryptonite
For a complete list of metahumans, see Category:Metahumans. Here is listed one example from each season.
- Season One: Ryan James had a brain tumor which gave him the ability to read minds. (Stray, Ryan)
- Season Two: Chrissy Parker had the ability to restore her youth with a kiss of death. It was not revealed how she obtained this ability, but she was actually born before the meteor shower. (Redux)
- Season Three: Jeremiah Holdsclaw received superhuman abilities from the Starblade, an ancient Kawatche artifact. (Talisman)
- Season Four+Five: Bart Allen received super speed from an accident involving a bright flash of light. (Run, Justice)
- Season Five+Six: Victor Stone received his powers after a car accident; he was taken to a cybernetics company and received biomechanical implants. (Cyborg, Justice)
- Season Seven: Dr. Curtis Knox attained immortality many centuries ago; the way he obtained it, however, is currently unknown. (Cure)
- Season Eight: Lana Lang put on the Prometheus suit that Lex had made and she gained super speed, super strength, invulnerability, and the ability to absorb kryptonite radiation. (Power, Requiem)
Metahuman since birth
Some metahumans' powers can be passed down to their offspring like a genetic trait.
- Season Two: Kyla Willowbrook's skinwalking abilities were passed down through her tribe. (Skinwalker)
- Season Four: Evan Gallagher aged rapidly. (Ageless)
- Season Five: Arthur Curry inherited his abilities from his mother. (Aqua) Maddie Van Horn inherited the ability to control glass from her father. (Fragile)
- Season Six: John Jones is naturally gifted as he is Martian and has his powers from birth. (Labyrinth)
- Season Eight: Davis Bloome was genetically engineered on Krypton by combining the DNA of General Zod and Faora with that of the planet's most violent and powerful creatures. (Bloodline)
- Season Nine: Icicle was a metahuman with the power to control ice. He inherited his powers from his father. (Absolute Justice)
- Season One: Whitney Fordman could walk through walls when he got a kryptonite tattoo. (Kinetic) Lana Lang became psychically linked to the first person who found her after a gas explosion covered her in meteor rocks. (Obscura) Harry Volk from the Smallville Retirement Center became young again temporarily when exposed to the Kryptonite in the pond outside. (Hourglass)
- Season Two: Eric Marsh used refined kryptonite like steroids for superior athletic performance. (Witness)
- Season Three: No one except Clark could lie to Chloe Sullivan after she was exposed to a kryptonite-based gas. (Truth)
- Season Four: Lana was possessed by her ancestor Countess Margaret Isobel Thoreaux numerous times. (Crusade, Spell, Sacred, Commencement)
- Season Six: With the use of a special drug, Oliver Queen temporarily developed a metahuman accelerated healing factor. (Rage)
- Season Seven: Lana had Clark's powers for only a short time until Clark took them back. (Wrath) Pete Ross chewed kryptonite-laced gum and developed stretching powers. (Hero)
- Season Eight: Chloe Sullivan was infected by Brainiac at the end of Season 7 and obtained all his knowledge and could analyze data at super speed. Her powers were removed by the Legion during (Legion)
- Season Ten: Lois had Clark's powers as a trial Jor-El had set out before them as a test of their relationship (Prophecy)
- "Metahuman" is a DC Comics term and did not appear on show until Season Nine's Absolute Justice.
- Chloe listed people who received their powers from meteor rock or other mutations as a different group than people born with powers, which she called "metahumans." (Warrior)
- Metahumans were more frequent, starting with the episode Hex with Zatanna being the first main metahuman to start adding more metahumans in Smallville. 
- The "Metahuman"s are very similar like the Dämons in the series "Charmed": Powerful, revenge-spirits.
- The term "metahuman" has been used to describe those affected by (and gained powers because of) the particle accelerator blow-up in the new CW series, The Flash, in the pilot.