*spoilers for if you are not up to date with either the show or infernal devices*
In the show you are introduced to yin fen as if it were any other recreational drug. Izzy gets hooked on it and displays the typical drug addict symptoms: cravings, fever, jitteriness, ect. She is shown to be addicted to it, she is willing to do anything to get more of it; she is shown to be a very typical, unflatteringly painted, drug addict.
This completely destroys and undermines Jem Carstairs’ entire character arc.
It is immediately established in Clockwork Angel that Jem is not a drug addict in the common sense. Yin fen is not a metaphor for meth or cocaine or any other recreational drug. It is a metaphor for the wasting, cureless diseases of the day, such as consumption or typhoid or something:
A hero […] who was condemned to die young of a fatal demonic illness, no matter how desperate the efforts were to save him, just as in reality victims of consumption sickened and died without penicillin(Forward of Clockwork Princess, pg. 4)
Clare states it clearly herself, yin fen is not a recreational drug like the show made it to be.
By giving Izzy this plotline, they have ruined any chance of Jem’s arc making any sense at all. People would see that Jem is addicted to yin fen and not be able to understand why he can’t just kick the habit. It wouldn’t make any sense that the drug is killing him, turning his hair and eyes silver and paling his skin, because this very obviously not what happens to Izzy. Izzy isn’t dying, she just feels like she is.
It is made very clear that Jem hates what yin fen has done to him. He hates that he must rely on it, he despises how it has stolen his life from him. And while he compares it to the Opium in China and himself to the addicts(thus offering a compelling metaphor about colonialism and racism):
The British bring opium into China by the ton. They have made a nation of addicts out of us. In Chinese we call it ‘foreign mud’ or ‘black smoke’. In some ways Shanghai, my city, is built on opium. It wouldn’t exist as it does without it. The city is full of dens where hollow-eyed men starve to death because all they want is the drug, more of the drug. They’ll give anything for it. I used to despise men like that. I couldn’t understand how they were so weak.
There was one thing they couldn’t fix, though. I had become addicted to the substance the demon had poisoned me with. My body was dependent on it the way an opium addict’s body is dependent on the drug.
(Clockwork Angel, ch. 15, pg. 339-340)
He also makes it very clear that the drug is more of an bastardized medicine:
After weeks of experimentation they decided that nothing could be done: I could not live without the drug. The drug itself meant a slow death, but to take me off it would mean a very quick one.
The yin fen is what keeps Jem alive, and he despises that. He wants to burn bright like Will does, he wants to live to grow old with Tessa(though not for her but that’s another rant). This why he throws it in the fire in Clockwork Princess, why he was taking less of it. He loathes relying on it.
This is not the case with Izzy. Izzy, like most drug addicts, craves how good the yin fen makes her feel. She actively wants more of it. It is not a unavoidable and cruel medicine, it is a recreational drug.
But the worst aspect of this is that it plays right into the negative and degrading view the other Shadowhunters have of Jem and further causes and creates Jem’s greatest fear.
The books works extremely hard to make it very clear that Jem Carstairs is not a drug addict. It is consistently referred to as his illness, the other characters work hard to combat this kind of thinking in the novels themselves. This plays into the vilification of the Lightwoods especially, with Gabriel constantly saying awful and derogatory things about Jem:
“You’re a decent Shadowhunter, James,” [Gabriel] said, “and a gentleman. You have your–disability, but no one blames you for that.”
(Clockwork Angel, ch. 9, pg. 206)
“I think,” Gabriel said, “that perhaps you might consider whether jokes about opium are either amusing or tasteful, given the…situation of your friend Carstairs.”
Will froze. Still in the same tone of voice, he said, “You mean his disability?
Gabriel blinked. “What?”
“That’s what you called it. Back at the Institute. His ‘disability’.” Will tossed the bloody cloth aside. “And you wonder why we aren’t friends.”
(Clockwork Angel, Ch. 11, pg. 269)
Not only this, but the scenes during and after Jem retrieves Will from the Drug Den, are extremely telling.
When Jem drags Will out of the den, the reader sees him lose his temper for the first time:
“You did not have to come and fetch me like some child. I was having quite a pleasant time.”
Jem looked back at him. “God damn you,” he said, and hit Will across the face, sending him spinning. Will didn’t lose his footing, but fetched up against the side of the carriage, his hand to his cheek. His mouth was bleeding. He looked at Jem with total astonishment.
(Clockwork Prince, ch. 9, pg. 195)
In this moment, Jem is so blindingly angry at Will, even Tessa observes herself how this was so utterly unlike him, because he feels as if Will is mocking Jem and his addiction by going and getting high on a drug when Jem is literally dependent and dying because of the yin fen.
“There’s no cure,” […] “I will die, and you know it, Tess. Probably within the next year. I am dying, and I have no family in the world, and the one person I trusted more than any other made sport of what is killing me.”
“He knows what it means to me,” he said. “To see him even toy with what has destroyed my life–”
(Clockwork Angel, ch. 9, pg. 200)
Because Jem has to battle against the label of a drug addict everyday, and his biggest fear is that he is just a addict, that that’s all anyone sees. He hates that label. Which, as seen, is openly talked about in the books. This is such a big deal that Will actually apologizes for it:
“I went to that den because I could not stop thinking about my family, and I wanted–I needed–to stop thinking,” said Will. “It did not cross my mind that it would look like I was making a mockery out of your sickness. I suppose I am asking your forgiveness for my lack of consideration.”
(Clockwork Prince, ch. 11, pg. 247)
Even though Will makes a point to never apologize about anything so that others will hate him. He apologizes to Jem for this thoughtlessness because he realizes how royally he messed up.
All of this is totally disregarded in Izzy’s storyline. People entering into TID after watching the show will be confused and not understand how Jem is sick and dying and is not really a drug addict at all. In short, they will enter into the novels with a prejudice and misunderstanding of Jem, and see him just like the other Shadowhunter’s do: a weak drug addict.
tl;dr:the show totally ruins and misconstrues and mocks Jem’s character arc by giving Izzy such a typical(and utterly incorrect) recreational drug addict storyline and I am furious about it.