Rachel over at 
Bellis Perennis has written a rather good blog post about the Leah Betts case, a teenager who dies in 1995 after taking 1 ecstasy pill and then drinking 15 pints of water.  It's a rather good post so I am going to crosspost it in it's entirety, with my comments after her words:
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The failed legacy of Leah Betts 
This is a thousand-word post about drugs and death. Sorry.
I was eleven when Leah Betts died and she has haunted me since. For  those of you who weren't eleven years old, who grew to maturity without  her icon hanging in the cluttered gallery of your subconscious, she was a  teenage girl from Latchenden, Essex. She lived eighteen years, took one  Ecstasy pill, drank fifteen pints of water and died.
Fifteen pints of water will do that to a person.
Leah's death became a cause célèbre throughout the country. Her  picture was printed full-page in every newspaper, shown nightly on the  news, pasted on billboards. It billowed above the roads on my way to  school, unavoidable, that failed ex-voto image of the girl-next-door  shrunken to a silkworm, spun around with so many life-support tubes that  they obscured her face and, even then, couldn't save her. A ghoulish  epitaph accompanied the image: 
Sorted.
Understandably, Leah Betts's parents wanted her death not to be  meaningless, to fulfill some purpose as their daughter's short life had  prevented her from doing. They had the support of the government, the  media and the schools and this powerful alliance went after the cause of  her death with a massive publicity campaign. Rather strangely in my  (adult) opinion, they went after the Ecstasy and not the water.
The story was a perfect one for this purpose. Leah seemed completely  normal, just pretty enough and just clever enough and, crucially, just  the right age for her fate to strike a chord with my cohort of awkward  adolescent girls. With the icon of the Blessed Leah before us -- struck  down so gruesomely and at such a romantic point in her life, the  threshold of freedom and adulthood -- we pledged to never to succumb to  the same fate.
Hmm. What I mean to say is, I can't have been the only one who was  terrified of dying the same way as her, despite having no desire to take  drugs in the first place, can I? The narrative was detailed in its  grisliness. The heat, the panic, the collapse. The vigil at her bedside.  At first we were told it must have been a bad pill -- 
it's so easy to get them, even when you think you know what you're  buying! Then the tests revealed that what she'd taken had been as advertised. 
Just one pill can do this to you! Sorted. They never mentioned the water. I didn't know about it until I was in my twenties.
One thing that did strike me as strange about the parable was that  Leah had taken her pill at a birthday party in her own house. In my  adolescent mind, as in the pamphlets pressed on us in school drug  awareness classes, Ecstasy was inextricably linked with nightclubs. I  had never been into one and did not intend to. Nightclubs were filled  with all of the things I hated: disorienting lights and noise, pop  music, dancing and -- most of all -- other people. As such, I felt  pretty sure I was also immune from the shadowy pushers who, the  pamphlets assured me, lined the walls of every club looking for  teenagers to poison. Why would you take it in your own home, even at a  party? Maybe that was her first mistake, after all. She had gone  off-script.
Now, of course, I think that was a pretty sensible idea. Much better  to try something new at home, where your kitchen and bedroom are and you  won't get lost, where your friends can look after you, than out in a  club. The tragedy was that her friends didn't know enough to look after  her. Water seems so safe. Drugs and dancing overheat you. Who would have  guessed that fifteen pints was too much, that she'd dissolve as  irrecoverably as an aspirin tablet?
I think it's disgusting that we weren't told what really killed her. I get angry, even now, that nobody told 
her.
People have been taking drugs of various kinds since we've been  people. Despite the best efforts of prohibitionists, they will probably  continue to do so for a long while yet. Even the ghost girls invoked by  the media, hovering mournfully in their phantom hospital gowns, can't  dissuade everyone.
It's ridiculous to tell young people nothing more than, "Don't do  it!" when we're talking about a recreational activity no more dangerous  than horse-riding, and criminal to lie about the ways in which they can  make themselves safer. You wouldn't send an eighteen-year-old on their  first horse ride without a helmet or bridle, would you? I'm not trying  to argue that Ecstasy use is without risks, simply that those risks  should be honestly set out by those taking it upon themselves to do so.
Leah Betts's pointless death could have been used to point out some useful facts about 
hyponatraemia (it's bad; it's avoidable) and 
SIADH  (not being able to pee enough, which can be caused by MDMA). That could  have been useful. Instead, they lied to us and terrified us with her  picture. Terror may work for a while, but when young people eventually  realise they've been lied to, any lessons that were supposed to go down  with the lies are usually discarded too.
In death as in life, she was a martyr to dishonest education.
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I wrote the above in my lunch break and mostly from memory; when I'd finished it, it was time to go and find some links. 
Leah's Wikipedia page is pretty good (warning: contains the photo of her in hospital). From there, I found 
this retrospective article on the BBC from 2005.
There's a 
long section about Leah and the rest of the "Public gallery of dead young women" used as propaganda tools against drug use in 
Chilling Out: the cultural politics of substance consumption, youth and drug policy  by Shane Blackman. I had no idea there were so many of them; I could  certainly only have named Rachel Whitear, whose post-mortem photograph  had a similar chilling effect on me at sixteen. I have lain awake on  more than one night thinking unhappily about her, despite never in my  life having had the thought that taking heroin might be a good idea. The  works of William S. Burroughs had put me off that many years before. 
Chilling Out looks pretty good. I might see if they have a copy in the library.
Here are some more things I didn't know but found out today: about  SIADH (biochemistry is fascinating); that, so much for the menace of 
ONE TABLET, Leah Betts had taken Ecstasy at least twice before; that the Times Style Guide still 
recommends spelling Ecstasy with a capital E, so I have done.
Wikipedia claims that the Sorted poster actually showed a picture of  Leah alive and smiling, but the only references linked to mention it as  showing the more sensational picture of her in the life-support machine,  the same as I remember. If Wiki is right, it's interesting that so many  people share the same constructed memory. I couldn't find any images on  Google to decide one way or the other.
Here's 
FRANK, the current  state of the art of government drug awareness initiatives. I've never  Talked To Frank so can't really recommend or unrecommend him, but there  he is for the sake of completeness.
Finally, here's the 
Erowid page on 
MDMA.  I haven't poked around much in there, either, but Erowid is generally a  fantastic resource so I feel comfortable recommending it.
Edited to add: I also meant to link to this report from the RSA, 
Drugs - Facing Facts. It's nice to have the RSA backing me up on this one. The publication itself is also an interesting read.
Many practitioners are convinced that close-range campaigns with a specific harm reduction message – safer clubbing, avoiding driving under the influence, the risks of heroin or methadone over-dose after leaving prison – are more likely to have a positive effect than highly visible general-deterrence campaigns like the largely unsuccessful ‘Sorted’ posters that featured photographs of Leah Betts as she was dying. Information presented neutrally and in a less alarmist form stands a better chance of convincing an audience that is naturally inclined to be cynical. (Page 163)
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My comments to Rachel on her post were as follows:
"Very well put.  I was 15 when Leah died, and I seem to recall my mother  using it as an excuse to tell me about the dangers of drugs.
This  didn't go down too well with me, (mostly cos I was 15) and when, at some  point later I did find out that it was the water that killed her, and  told my mother this, (and possibly other adults too) I believe they  ignored me as dying from water consumption doesn't fit the anti drug  narrative.
This, I suspect, is why you didn't find out about it till  you were an adult.  It does piss me off when people don't tell the truth."
It does piss me off when people aren't honest about dangerous substances.  I have never really seen the appeal of drugs (beyond weed), but once took half an e and proceeded to get depressed.  So, that sort of thing isn't for me - not because I think it's dangerous, I'm just not interested.  I'd still like the truth though.