Warm Bloodbags Are Everywhere

Some characters have it rough, especially those that get infected with a Viral Transformation that gave them a Horror Hunger for human. They may be forced to feed on humans more than they'd like, even becoming good at it, much to their own shame. Or they may try to be a Vampire Refugee or Friendly Neighborhood Vampire, werewolf, or whatever, and only feed on non-sentient things.

Then they make a Muggle friend, maybe even with Love Interest potential! This is all sorts of awesome since it helps reconnect them to humanity and makes their existence bearable, maybe even allowing a semblance of a normal life/relationship.

There is, however, the obvious problem that they are now underfeeding and spending a lot of time with someone who is directly below them on the food chain. Things will only get worse before something goes Horribly Wrong. What usually happens is they suffer a brief bout of Glamour Failure when the Horror Hunger is strongest, they'll reflexively extend their fangs and claws, start seeing in Vein-O-Vision, or put on their Game Face. However they'll manage to retract them through sheer willpower before their new Delicious Distraction friend/love interest notices... most of the time.

Where they usually fail to maintain the Masquerade is when facing a situation where their new friend is bleeding and they're desperately hungry, and to avoid attacking their friend they give in to the lesser urge for the spilled blood and start lapping it up. Or to have their friend show them a cross/garlic/open flame and for them to recoil in horror before hissing and fleeing (when not spontaneously combusting). At that point, the gig is up and they usually stay away in shame at being exposed or for fear of losing control completely.

On the bright side, the friend does usually prove to be very loyal and finds them afterwards, and decides to take their chances by remaining close.

Contrast Emergency Transformation. Compare Frequently-Broken Unbreakable Vow. Often somewhere in the middle or low end of the Sliding Scale of Vampire Friendliness.

Anime and Manga

 * This is basically the entire plot of Vampire Hunter D: Bloodlust.
 * Also happened in the original Vampire Hunter D. One time when D was with Doris Rumm, his fangs extended and he had to fight to control himself. Watch it here, starting at about 2:00.

Fan Works

 * This is essentially the entire premise of the fanfiction story For Love Or Blood... the trope, however, is tragically subverted in that Jericho, the protagonist, isn't able to restrain his Horror Hunger when his first Love Interest is injured, and ends up killing her. Angst ensues.
 * In The Return Misako (part of a brood of Friendly Neighbourhood Succubae) has to be reminded several times not to refer to her extended human family (and humans in general) as "meatbags".

Film

 * Treated semi-humorously in Shadow of the Vampire, where Count Orlock appears as Max Shreck, an actor impersonating a vampire and Murnau has to prevent him from preying on the staff and co-stars at almost every turn.
 * Let the Right One In has this happen towards the middle. Eli follows Oskar to a secluded spot after she hasn't fed for a few days, and Oskar unexpectedly cuts his palm and offers to be blood brothers with her. She manages to fight the urge to eat him by lapping up the blood he was spilling, but reveals her Game Face in the process, growls, and runs away.
 * Happened in the first Madagascar when Alex realized all his friends were, in the wild, basically his food. Cue Meat-O-Vision.
 * Early in the Interview with the Vampire adaptation, Louis feels tremendous shame and guilt over feeding on people, so he goes as long without drinking blood. Ironically, this means that during these periods anyone close to him is in tremendous danger of him starting to feed upon them without him intending or realizing it.
 * Freddy in Return of the Living Dead, anguished by the pain of decomposition, loses it and attacks his girlfriend Tina, desperate to eat her brain.
 * Twilight, big time.

Literature
"Alice: Edward, it's like you're a recovering drug addict, that's dating crack. It's never gonna work."
 * This is the entire plot of Twilight.
 * This is lampshaded in the How It Should Have Ended entry for Twilight.

"Elfangor: You're going to have to fight the hunger. Arbron: What, are you afraid I'll morph and try to eat you? Elfangor: Yes, Arbron. I am afraid."
 * Happens to Ivy multiple times when around Rachel in the early books in The Hollows series.
 * Discworld:
 * Played for laughs in The Truth: Otto Chriek is a "Black Ribboner" (member of an organisation of reformed vampires) and a woman faints in front of him. The image of a fainting woman with "bosoms going in and out and up and down like that" triggers the vampiric urges and he panics, and ends up singing jolly songs about not wanting to drink blood ("zer drink zat's in zer livink vein is not zer drink for me ...") until someone provides him with non-human blood.
 * Alluded to for Angua in Feet of Clay, although she seems to have her werewolf instincts well under control, and conceals them from Cheery more because she's afraid the truth will harm their budding friendship than for fear she'll actually hurt the Watch's new recruit. Angua herself admits that it's not a matter of not wanting to [eat people], it's a matter of wanting to and not doing it.
 * The Star Trek: The Original Series novel First Frontier featured the Clan Ru as enemies. Essentially, they're sapient velociraptors (the cinematic version). They find it nearly impossible to live in peace with the humanoid mammals and quasi-mammals that dominate the galaxy for this reason. (And prompt Kirk to wonder how seriously he'd be able to take people who smelled like warm baking bread.)
 * Taxxon morphs in Animorphs are like this.


 * Several times, Harry Dresden notes that Thomas, a White Court vampire, is getting very hungry. However, until Turn Coat, Thomas was able to keep his Hunger in check.
 * His family refers to normal humans as "cattle," demonstrating exactly what the Hunger makes you feel like.
 * The same but more so with
 * Thirsty, as the protagonist turns into a vampire.
 * The first phase of being transformed into a vampire in the Night Huntress books is a horrific uncontrollable surge of the normal blood thirst present in all vampires. Most vampires manage to get back normal control after around a week in restraints being fed on bagged blood and attended by undead. managed to get to the point where living people were allowed into the room after about a day.
 * In Diario de un Zombi, Erico is a zombie trying to ferry two living humans to the alps. The flesh eating pangs slowly grow over time, adding him to the threats they face.

Live-Action TV

 * Mitchell in Being Human (UK) had to do this practically every other episode, with mixed results.
 * M'lee, the calcium-eating alien in season 1 of Farscape did this, having the light-emitting horn crown on her head change color when hungry for calcium. Interestingly, she managed to "fang off" after her cover was blown to plead with Crichton that she isn't evil, but she needs calcium to live and (being trapped on a forest asteroid with no other animals) has to find a humanoid source to eat. Happily, she isn't killed and they manage to find her a suitable substitute to eating the main cast... the other villain of the week and the Peacekeepers arriving to investigate. It's implied that she's now being employed by Scorpius in exchange for the disloyal members of his crew.
 * M'Lee's plotline was apparently going to come to some sort of larger fruition later on, had the series continued. Sadly...
 * Seen in the pilot for the television adaptation of The Vampire Diaries when the lead vampire is around his bleeding Love Interest.
 * Seen again in episode 2.03 when  started bleeding and   just couldn't resist taking a bite.
 * Stefan is noted as having a problem with blood addiction. His use of animal blood, while most non-homicidal vampires steal from blood banks, is because he's afraid of relapse. Later on he starts to carefully start drinking human blood again, as using only animal blood makes him weaker.
 * Angel sometimes has this problem in Buffy the Vampire Slayer and later in his own series. He's mostly overcome the issue by using animal blood, but flashbacks to his early attempts at living around people show it wasn't always so easy for him to resist humans, and if he's severely weakened or starved, he's liable to grab and start drinking from someone before he even realizes what he's doing.
 * Other vampires in the Buffy Verse have the same problem, though being soulless demons means that they usually don't even try to restrain themselves. One exception is Harmony, who joined the good guys in season 5 of Angel and, of course, wasn't allowed to feed on humans. She complained that, since she doesn't have a soul like Angel, it's twice as hard for her to resist eating people.
 * In the second season, Spike even lampshades this by stating outright that one of the reasons he doesn't want to destroy the world is "Billions of people walking around like Happy Meals with legs." He also doesn't like how Angel is stealing Drusilla, but that's beside the point.
 * Rev Bem in Andromeda is always having to stop himself eating and laying his young inside his crewmates. He relies on his religion ("Wayism") to get him through the day.
 * An odd, non-predatory and character-specific example comes in The Sarah Connor Chronicles, where Cameron admits to John Connor that, deep down, her hard-wired programming is permanently directing her to kill him at all times, and is overridden only by the other programming that the future John Connor used on her. At one point, Cameron does revert and starts trying to kill John, only to override her own termination directives at the last second. Cameron also has to fight the opposite reaction when dealing with people she interprets as threats to John, and also admits that she doesn't know whether she will kill someone who she views as a threat. Her uncertainty regarding both of these eventually drives her to wire up an explosive device into her skull in case she loses control and needs to be put down.
 * Lyekka of Lexx was very matter-of-fact about this: she'd "prefer" not to eat her human friends, but deprived of other food sources, she would.
 * An episode of The X-Files featured a brain-eating mutant who tried not being a brain-eating mutant.
 * Oddly simultaneously inverted and played straight in Supernatural. Sam is a demon-blood-tainted human who became addicted to demon blood and the primarily-demon-exorcising power that it provides. Withdrawal kind of sucks when your main target as a hunter is demons and your weapon of choice is a knife. And then the demons use this advantage and make him drink their blood. It doesn't turn out well for them.

Tabletop Games

 * In Vampire: The Masquerade, younger vampires can feed on animal blood in lieu of human blood, but it's not nearly as satisfying. Unfortunately, that means this trope occasionally comes into play, because every once in a while a neonate attempts to maintain relationships from their mortal lives while also trying not to feed on humans. Sad to say, when this inevitably happens, it is treated as a breach of the Masquerade and the bloodbag in question needs to have their memory erased or be killed before they can spread the word.
 * This trope is mechanically enforced in both Masquerade and the successor game Vampire: The Requiem, as a vampire low on blood will often have their body taken over by the Beast, who doesn't care where the blood comes from as long as it gets fed.
 * The Achilles' Heel of Vampire teams in Blood Bowl: Vampires have a 1/6 chance each turn to run off to seek blood to feed on instead of playing the game. This leads to them either draining one of your Thrall players (leading to an instant KO) or running off the pitch to hunt audience members (cancelling the action and causing a turnover).

Video Games

 * Arcueid in Tsukihime. Normally she's able to suppress her bloodlust by sheer power, but when events in the story force her to use much of it up, she's left relying mostly on willpower. Despite knowing this, protagonist Shiki Tohno chooses to stick with her. Satsuki is another example for whom things didn't end as well.
 * Digital Devil Saga. Every enemy, potentially, can be easily devoured. Mick the Slug is quite fond of eating his own men.
 * In Soul Reaver you can choose to live peacefully inside the Human Citadel. To survive, Raziel can take small sips of energy from the friendly locals (if you weren't a Complete Monster and killed off their fellow vampire hunters or neighbors, that is). Be a glutton though and the host dies.