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The spell Reducto is only used four times in the books and never on a person but it generally just blasts solid objects out of the caster's path, so I think there is no canonical answer for this question. But lets try to answer this from the different occasions from where it is used.

• Harry learned this spell as part of his preparation for the third task. He used it to blast a hole in the hedge.
  As the spell created hole in the Solid hedge, we can assume that the spell can knock a person down by damaging his solid bones.


• A Reductor Curse during D.A. training reduced a table to dust. This makes the spell even capable to reduce the bones of target to dust,


• Used by the various D.A. members at the beginning of the battle of the Department of Mysteries to smash shelves in the Hall of Prophecy as a diversion.


• 
  

  Snape used this on rosebushes to blast them apart at the Yule Ball after his talk with Karkaroff. As the Reducto curse applies to thin rose bushes, we can tell that it can reduce even skin to dust.

  
  
  

  But whether it applies to organs cannot be answered based on the occasions from which it is used in books.

  
  

To the best of my knowledge, there is, in fact, one reported incident of a friendly fire hit of the Reductor Curse used by Luna Lovegood indirectly on Ginny Weasley, which broke her ankle.

During the Battle of the Department of Mysteries, in the Order of the Phoenix chapter Beyond the Veil, Luna reports the incident to Harry as follows.

(Spoilers. Duh.)

"I think her ankle's broken, I heard something crack," [...] "Four of [the Death Eaters] chased us into a dark room full of planets; it was a very odd place, some of the time we were just floating in the dark -" [...] "-anyway, one of them grabbed Ginny's foot, I used the Reductor Curse and blew Pluto up in his face, but..."

Luna gestured hopelessly at Ginny, who was breathing in a very shallow way, her eyes still closed.

Now, it isn't explicitly stated that it was the blast from the curse hitting the planet that broke Ginny's ankle, but it seems unlikely that her ankle would have been broken only from being grappled by the Death Eater. If the damage was caused by the curse, this would suggest that the shockwave alone from the Reductor Curse is enough to break bones. If directed at the head instead of the foot, it is possible that the blast would be forceful enough to snap the neck or shatter the skull, possibly causing paralysis or death.

Assuming that it was the struggle that broke Ginny's ankle and not friendly fire, we can see that the indirect blast to the head was still powerful enough to incapacitate a fully grown adult male, indicating that a direct hit would have done more damage still.

Almost every spell in the Harry Potter universe can be used to kill someone, and don't forget that Ginny uses a Reducto in the films and it does hit a Death Eater. I did a quick count of the number of Death Eaters at the start of the fight and when Harry's friends are defeated by them. There were 8 to start with and at the end I only counted 6. Since the only potentially deadly spell we saw cast was Reducto and we only saw one Death Eater incapacitated before that, it's safe to asume that the Death Eater was either killed or very badly injured.

I would argue that it would probably not have much of an effect on humans.

The information about the Reductor Curse is somewhat limited, as there only appear to be seven mentions of it in the entire series:

• 
  

  Goblet of Fire Chapter Thirty-One

  
  
  

  
  

  Harry had soon mastered the Impediment Curse, a spell to slow down and obstruct attackers; the Reductor Curse, which would enable him to blast solid objects out of his way; and the Four-Point Spell, a useful discovery of Hermione's that would make his wand point due north, therefore enabling him to check whether he was gong in the right direction within the maze.

  
  

  
  


• 
  

  Goblet of Fire Chapter Thirty-One

  
  
  

  
  

  He turned back, took a right turn, and saw an odd golden mist floating ahead of him.

  
  
  

  Harry approached it cautiously, pointing the wand's beam at it. This looked like some kind of enchantment. He wondered whether he might be able to blast it out of the way.

  
  
  

  "Reducto!" he said.

  
  
  

  The spell shot straight through the mist, leaving it intact. He supposed he should have known better; the Reductor Curse was for solid objects.

  
  

  
  


• 
  

  Goblet of Fire Chapter Thirty-One

  
  
  

  
  

  The air was suddenly full of Cedric's yells. Horrified, Harry began sprinting up his path, trying to find a way into Cedric's. When none appeared, he tried the Reductor Curse again. It wasn't very effective, but it burned a small hole in the hedge through which Harry forced his leg, kicking at the thick brambles and branches until they broke and made an opening; he struggled through it, tearing his robes, and looking to his right, saw Cedric jerking and twitching on the ground, Krum standing over him.

  
  

  
  


• 
  

  Order of the Phoenix Chapter Nineteen

  
  
  

  
  

  He and the D.A. were resisting her under her
  very nose, doing the very thing that she and the
  Ministry most feared, and whenever he was supposed
  to be reading Wilbert Slinkhard’s book during her
  lessons he dwelled instead on satisfying memories of
  their most recent meetings, remembering how Neville
  had successfully disarmed Hermione, how Colin
  Creevey had mastered the Impediment Jinx after
  three meetings’ hard effort, how Parvati Patil had
  produced such a good Reductor Curse that she had
  reduced the table carrying all the Sneakoscopes to
  dust.

  
  

  
  


• 
  

  Order of the Phoenix Chapter Thirty-Five

  
  
  

  
  

  Five different voices behind him bellowed “REDUCTO!”
  Five curses flew in five different directions and the
  shelves opposite them exploded as they hit. The
  towering structure swayed as a hundred glass
  spheres burst apart, pearly-white figures unfurled
  into the air and floated there, their voices echoing
  from who knew what long-dead past amid the torrent
  of crashing glass and splintered wood now raining
  down upon the floor —

  
  

  
  


• 
  

  Order of the Phoenix Chapter Thirty-Five

  
  
  

  
  

  “I think her ankle’s broken, I heard something crack,”
  whispered Luna, who was bending over her and who
  alone seemed to be unhurt. “Four of them chased us
  into a dark room full of planets, it was a very odd
  place, some of the time we were just floating in the
  dark —”

  
  
  

  “Harry, we saw Uranus up close!” said Ron, still
  giggling feebly. “Get it, Harry? We saw Uranus — ha
  ha ha —”

  
  
  

  A bubble of blood grew at the corner of Ron’s mouth
  and burst.

  
  
  

  “Anyway, one of them grabbed Ginny’s foot, I used the
  Reductor Curse and blew up Pluto in his face, but...”

  
  
  

  Luna gestured hopelessly at Ginny, who was
  breathing in a very shallow way, her eyes still closed.

  
  

  
  


• 
  

  Half-Blood Prince Chapter Twenty-Seven

  
  
  

  
  

  But at that moment, there were renewed sounds of scuffling from below and a voice shouted, 'They've blocked the stairs — Reducto! REDUCTO!'

  
  

  
  

From these passages we can derive several things. First we see that Reducto only works on solid objects. I would think humans are considered solid, but I don't know if they're considered objects, or even if the wording was meant to be precise enough to make such inferences. Specifically, we see two instances where the curse had no effect at all. It didn't do anything to the mist which was not solid, and it didn't do anything to the Astronomy Tower barrier which doesn't seem to have been physical at all.

We also see different levels of destruction. The original description just says that the curse "blasts solid objects out of [the] way" which doesn't necessarily entail any destruction at all. When used on the hedge in the maze it seems to have caused destruction, but on a very small scale (though perhaps the hedges were magically reinforced). Yet in the Order of the Phoenix passages it causes full-fledged destruction, entirely destroying a table, exploding shelves, and blowing up Pluto.

In any case we never see it used on a human, or as part of a duel at all. If it was possible to use it on humans in the same way as it is used on inanimate objects we might expect it to be useful in duels. While the good-guys might not have used it because they seem to usually actively want to not harm their enemies, it would certainly be useful for Death Eaters who have no compunctions about seriously injuring or killing their enemies, and it seems like a much easier curse to produce than some of the serious dueling spells.

Additionally, there are several other spells that, like Reducto, seem designed for inflicting some form of destruction on objects and are also never used against humans. For example, Diffindo is used for cutting things, and Confringo is used for exploding things. Diffindo is mentioned five times:

• 
  

  Goblet of Fire Chapter Twenty

  
  
  

  
  

  This gave Harry an idea. Pausing at a distance from them, he pulled out his wand, and took careful aim.

  
  
  

  "Diffindo!"

  
  
  

  Cedric's bag split. Parchment, quills, and books spilled out of it onto the floor. Several bottles of ink smashed.

  
  

  
  


• 
  

  Half-Blood Prince Chapter Eleven

  
  
  

  
  

  He pulled the old copy of Advanced Potion-Making out of his bag and tapped the cover with his wand, muttering, "Diffindo!" The cover fell off. He did the same thing with the brand-new book (Hermione looked scandalized). He then swapped the covers, tapped each, and said, "Reparo!"

  
  

  
  


• 
  

  Deathly Hallows Chapter Nine

  
  
  

  
  

  "D-diffindo," she said, pointing her wand at Ron, who roared in pain as she slashed open the knee of his jeans, leaving a deep cut. "Oh, I’m so sorry, Ron, my hand’s shaking! Diffindo!" The severed ropes fell away.

  
  

  
  


• 
  

  Deathly Hallows Chapter Thirteen

  
  
  

  
  

  “Did I?” muttered Harry, tugging at the chains binding her arms, “Well, I’ve had a change of heart. Diffindo!” Nothing happened. “Hermione, how do I get rid of these chains?”

  
  

  
  


• 
  

  Deathly Hallows Chapter Nineteen

  
  
  

  
  

  "Diffindo." It cracked with a sound like a bullet in the silence. The surface of the pool broke and chunks of dark ice rocked on the ruffled water.

  
  

  
  

We see clearly that Diffindo is powerful enough to entirely slice open Cedric's bag, entirely sever ropes, and chop through ice. (It didn't do anything to the chains, but the chains might have been charmed to not be severable.) In terms of physical force that should be powerful enough to do significant damage to a human. Yet we never find it being used deliberately on a human. The one time it was used accidentally on a human, all it did was cause a deep cut. This might indicate that the spell is fundamentally not meant for human use. Consider that it would certainly have been useful if it could have been used the same way on humans as on objects. For instance, Wormtail could have used it to sever his hand instead of using a dagger. (Though cutting through solid bone might be harder. But once we deal with that, I doubt Wormtail's dagger swing would have been powerful enough to slice straight through the bone.)

Additionally, in two of the cases above, Diffindo was used on something that was attached to a human, and the casters didn't seem at all worried that if the spell was off by a centimeter it could seriously injure the person.

Finally, if Diffindo could be used against humans the same way as it is used on objects then Sectumsempra would essentially be the same spell. As it seems that Sectumsempra is unique (and not just a slightly sharper "blade" than Diffindo), this again might indicate that spells can be specifically for or not for human use. (I.e. the difference between Diffindo and Sectumsempra is that the former is for slicing objects while the latter is for slicing humans).

Confringo is used twice:

• 
  

  Deathly Hallows Chapter Four

  
  
  

  As they soared upward, away from the two remaining Death Eaters, Harry spat blood out of his mouth, pointed his wand at the falling sidecar, and yelled, "Confringo!" He knew a dreadful, gut-wrenching pang for Hedwig as it exploded; the Death Eater nearest it was blasted off his broom and fell from sight; his companion fell back and vanished.

  
  


• 
  

  Deathly Hallows Chapter Seventeen

  
  
  

  
  

  The snake lunged as he took a running leap, dragging Hermione with him; as it struck, Hermione screamed, "Confringo!" and her spell flew around the room, exploding the wardrobe mirror and ricocheting back at them, bouncing from floor to ceiling; Harry felt the heat of it sear the back of his hand.

  
  

  
  

Here again we never find the spell being used against humans, despite the fact that an explosion spell be quite useful. This again may indicate that you simply can't explode a human being. At most you can do what Harry and Hermione did and explode objects in near proximity to your target.

Furthermore, note the following passage from Chapter Six of Order of the Phoenix:

“What d’you reckon it is?” said Harry.

“Could be anything,” said Fred.

“But there can’t be anything worse than the Avada
Kedavra curse, can there?” said Ron. “What’s worse
than death?”

“Maybe it’s something that can kill loads of people at
once,” suggested George.

“Maybe it’s some particularly painful way of killing
people,” said Ron fearfully.

“He’s got the Cruciatus Curse for causing pain,” said
Harry. “He doesn’t need anything more efficient than
that.”

There was a pause and Harry knew that the others,
like him, were wondering what horrors this weapon
could perpetrate.

Notice how they seem to think that Avada Kedavra is very unique. They don't seem to be aware of any other curses for killing, killing painfully, or killing multiple people at once. If curses such as Reducto, Diffindo, and Confringo could be equally used on humans, they surely should have realized that blowing someone up, blasting them apart, or slicing them, are ways of killing, killing painfully, and perhaps even killing multitudes. This, then, would support the contention that these spells cannot be used on humans the same way they are used on objects.

Thus, I would argue that there may simply be different classes of spells. Some are designed for causing harm to humans, while others are designed to cause some kind of physical destruction to objects. Just like the standard dueling spells don't work the same way on objects as they do on people (e.g. Voldemort's Avada Kedavra on the statue in the Ministry), it is possible that the non-dueling spells don't work the same way on people as they do on objects. They might still have some effect, like Hermione's Diffindo on Ron, but it probably wouldn't be too serious.

Reducto

The Reductor Curse enables the caster to blast solid objects out of their way.

_^Reducto

There are three Unforgivable Curses.

Now... those three curses - Avada Kedavra (Killing Curse), Imperius, and Cruciatus - are known as the Unforgivable Curses. The use of any one of them on a fellow human being is enough to earn a life sentence in Azkaban.

_{^{Harry Potter and the Goblet of Fire, Chapter 14, The Unforgivable Curses}}

The Avada Kedavra (killing curse) gives pain free death.

As the name suggests, the curse kills immediately and painlessly. It looks like a long flash of green light.

_{^{Avada Kedavra}}

If Reducto had the same affect on humans as it has on objects. Reducto has capability to cause a very painful death and yet it is not in the list of Unforgivable Curses.

If the curse was useful on Humans, the death eaters would have used it. I don't think it works on humans.

I was interested in this question too.

In the books/movies and in the HP games Reducto can blast through solid objects. In the 5th movie, Ginny first uses Reducto to blast a Death Eater statue in the room of requirement to dust. Then again in the department of mysteries. So based on the fact that Reducto can obliterate solid objects I would think it could obliterate a person as well.

Also in the 7th book during the fight between Mrs Wesley and Bellatrix, Mrs Wesley disintegrates Bellatrix in the great hall killing her. So maybe Reducto can kill. As to an earlier point about nobody using Reducto to kill people might be because Avada Kedavra is unblockable and kills someone instantly, whereas with Reducto it can be blocked and unless you hit someone dead on, you might blast off an arm or a leg.