The world of Harry Potter is full of dangerous potions - and many of them are treated in a surprisingly casual way. Indeed, Potions classes often teach students how to create some truly horrific substances.
Perhaps there's an argument to be had that brewing these potions is essential defense against many of the terrible wizards and creatures of that world. Still, it would be very easy to be caught unawares by one of the following concoctions, particularly if an antidote isn't at hand.
Veritaserum
Just a few drops of this truth potion could have an unsuspecting victim blabbing their innermost secrets. Harry Potter fans have long wondered why accused dark wizards weren’t fed Veritaserum, especially after Dumbledore memorably used it to extract Barty Crouch Jr’s testimony in Goblet of Fire. Just because it was used by a heroic character, however, does not make it right. Truth is subjective to the person telling it, and this is a world in which memories can be modified.
Ultimately, there are some truths that are best left private. In Order of the Phoenix, Umbridge attempts to slip Veritaserum into Harry’s drink. Had she succeeded, he would’ve spilled the location of a hunted innocent man (Sirius Black) and ruined his own attempts to rebel against her authoritarian decrees.
Emerald Potion/Drink of Despair
Unlike many potions on this list, a few drops of the Emerald Potion won’t have a huge effect. There’s little worry of having it slipped into one’s drink and being inundated with despair. However, drinking a lot of it results in extreme thirst, despair, and nightmarish hallucinations that attack the drinker’s soul.
Voldemort hid a Horcrux behind one of these potions, and Dumbledore had to drink it to retrieve the item. Ultimately, Voldemort’s plot here was to create a situation in which someone would knowingly subject themselves to its effects. In that sense, this might be one of the cruelest potions - a form of torture that the victim has to agree to.
Death Potion
This one’s in the name. The Death Potion is a highly corrosive potion that burns its victim in seconds. Simple and to the point. It does have an additional function though - being able to replay the victim’s memories to leave them in a delusionally happy state.
For this reason, the Death Potion was used by the Magical Congress of the United States of America for a time. In Fantastic Beasts and Where to Find Them, we see Newt Scamander nearly succumb to it. In that sense, the Death Potion was used as an instrument of state justice - the wizard version of the death penalty.
Amortentia
There are many love potions in the Harry Potter universe. Ron accidentally ingested one in The Half-Blood Prince and briefly became infatuated with Romilda Vane. This ruined his current relationship (not that he minded), and he even punched Harry for “disrespecting” Romilda. Despite its dubious effects, Ron’s mother happily admitted to brewing a love potion at school, and his brothers sell some as merchandise at their Weasleys’ Wizard Wheezes shop.
Amortentia is the most potent love potion of all and was memorably used by Merope Gaunt to trick and steal away Tom Riddle for years. This coerced union was responsible for the birth of Voldemort. Even ignoring that nasty outcome, this is a sinister level of control over another human being.
Felix Felicis
At first glance, the “Liquid Luck” potion doesn’t sound especially dangerous. All that it does is make the drinker “lucky” by twisting the circumstances of their day in favorable ways. For Harry in Half-Blood Prince, this enables him to extract a memory from Professor Slughorn through manipulation and sheer confidence. Otherwise, the only obvious issue with Felis Felicis is its ability to ruin all competitions - imagine Quidditch spoiled by a Seeker grabbing the Snitch immediately!
No, the issue is that reliance on Felix Felicis can cause overconfidence. Not every situation can be resolved by luck, and walking into a dangerous trap unprepared is a bad idea. Recklessness can be very dangerous, like a shot of foolhardy testosterone.
Garrotting Gas
Despite its name, Garrotting Gas is indeed a potion. Its fumes are the dangerous part - they have the capacity to choke and suffocate anyone who inhales them. All that a dark wizard would need to do is brew a flask and throw it upon the ground to create an environment that’d garrotte anyone who happened to walk nearby.
As with many dangerous potions in this world, it seems to be treated with less seriousness than might be expected. Fred and George Weasley were planning to unleash some in the school corridors to cause trouble for Dolores Umbridge. Apparently, even students can brew something this hazardous.
Forgetfulness Potion
The Forgetfulness Potion operates just like the spell Obliviate. The most known usage of this spell was by Gilderoy Lockhart, who cast it upon people to claim their achievements as his own and create a false career as a dashing adventurer. Another example is the fate of Bertha Jorkins, whose memory was cursed so thoroughly by Barty Crouch Sr. that her overall ability to retain memories was drastically reduced. She went from a gossipy know-it-all to dotty and unfocused.
The Forgetfulness Potion, then, operates in the exact same way. While not as immediate as Obliviate, it may be easier to pull off instead of shouting a spell at someone. Slipping a Forgetfulness Potion into someone’s drink could sneakily force them to lose their memory - to the point that they may not even remember drinking it!
Polyjuice Potion
Acquiring ingredients for Polyjuice Potion is already creepy enough. The brewer has to take “body parts” - e.g. toenail clippings and hair - from their intended target. In Goblet of Fire, Barty Crouch Jr stows Mad-Eye Moody in a trunk for an entire year, extracting his hair. This is part of a master plan to impersonate Moody and send Harry to his death.
This is the true danger of Polyjuice Potion - identity theft is a serious crime. Anyone impersonating another person can do whatever they wish in that body. Want to spy on someone by pretending to be their significant other? Want to commit murder in public and blame it on someone else? Transform into that person, and ruin their life.