A classical Not What It Looks Like scenario; Alice walks in Bob's room, sees Bob injecting something in his arm with a syringe. Assumptions are made, and conclusions jumped to, until Bob reveals that he's diabetic and was taking his insulin shot. May be played for drama or for comedy.
Pills and powders are also often mistaken for recreational drugs. Sometimes Mistaken for Junkie is used to criticize the Drugs Are Bad hysteria.
Unfortunately Truth in Television, and some groups are almost guaranteed to have encounters with it: diabetics, as mentioned, (due to the need for insulin injections); transgender people (due to the need for hormone injections and the status of androgens/testosterone and some estrogens as controlled substances); nocturnals or those with sleep phase disorder or those with hyperthyroidism as illegal users of stimulants (due to different sleeping patterns, and in the latter two, behavior that sometimes resembles stimulant users). Anyone who frequently has to have IV injections or blood work done will almost always get this, especially if they were unlucky enough to have encountered someone doing the injection/blood work so improperly as to give them a track mark. Legitimate pain patients and those legitimately taking a lot of psychotropic medications for mental illnesses are also often assumed to be junkies, because they're taking lots of pills/other medications, never mind that is to keep them alive and (somewhat) well.
See also Mistaken for Subculture.
In Pretty Woman, when Richard Gere enters the bathroom and finds Julia Roberts apparently swallowing something, he thinks she's popping drugs, when in fact she's just flossing her teeth.
In Mad Money, two of the protagonists see the third's syringes fall out when she drops her purse. They give her a mini-lecture on drugs, but of course it turns out she's simply diabetic. Except in this case she never tells them.
In the Disney Channel film Go Figure, Shelby runs off after being berated after falling and messing up her routine. Caitlin follows her into the bathroom and sees a syringe fall under the stall, and she begins to lecture her on drugs. Shelby tells her that she's diabetic. And then they go get sugar-free frozen yogurt.
In the Marlon Waynes movie Senseless, Waynes' character is using an experimental drug to enhance his senses to superhuman levels (in order to both get the girl and win a coveted business internship). His roommate walks in on him after he's given himself an injection, sees the needle, and (based on this and Waynes' erratic behavior) assumes that he's gotten hooked on heroin. Eventually Waynes' character explains what is really going on to his roommate, but its not clear if he really believes him; during the big confession scene during the climax as Waynes reveals that he's been using a drug to get an advantage in the competition for the internship, the roommate interrupts him to announce that "It's NOT heroin!"
Only a partial example since Sherlock Holmes actually is canonically a drug addict, however, there's one particularly amusing scene in The Adventure of the Missing Three-Quarter where Watson walks in on Holmes filling a needle during a case and panics. It turns out that the solution is really linseed oil, with which Holmes marks a carriage so that Watson's dog can follow it.
One of the Babysitters Club books featured this, in which diabetic Stacey had trouble getting her insulin kit past security at a concert.
One of the plot points in the YA series-pulp book The Real Deal: Unscripted.
There's a variation in one of the Trace novels by Warren Murphy. A bigtime movie star travels with a doctor who gives him pills on a regular basis. The movie star acts like these are some kind of "happy pills", but the doctor later reveals that the star is in poor health and all the pills are actually medically necessary.
In The Bible, Hannah's impassioned prayer (doing so almost silently) is mistaken for drunken ramblings by Eli, a head priest (who should really know better).
Similarly, the disciples are mistaken for drunk during the Pentecost, when the Holy Spirit pours out the gift of tongues on them.
In the former's case, however, the last chapter or so has made it clear that the behaviour Eli suspected was common at this time. He probably caught more than one person wandering into the temple drunk, and was making an innocent mistake.
Sadly, in the case of the latter, there are preachers who ran with the idea that Peter and the other apostles were actually "drunk in the Spirit" based on a misinterpretation of Peter's defense, "For these are not drunk, as you suppose, since it is only the third hour of the day," (Acts 2:15) and hopscotching it with Paul's admonition in Ephesians 5:18 to "do not be drunk with wine, in which is dissipation; but be filled with the Spirit." In this case, they are being Mistaken (by the very readers of Scripture) For Junkie Prophet.
Max was caught taking a supplement that stopped her seizures on Dark Angel and her roommate thought it was drugs, so she flushed the pills.
Max herself saw entered the exact moment a prostitute was injecting herself and received a very casual "I'm a diabetic, this is my insulin shot" right after the woman noticed her presence.
There's a scene on House which fits this: the title character is snorting a white powder, and it turns out to just be antihistamines for his cold.
Again on House, in one episode House stopped taking his vicodin and appeared pain free. His colleagues deduced he had started heroin but he had actually turned to methadone.
This trope is what actually attributed to the loss of House's leg muscle. He was in obvious huge amounts of pain, injected himself in the thigh with demerol, the Doctors thought he was just an addict and sent him home. And you know what happened next...
There's an example on Jonathan Creek where something like this is pulled on the audience - we see the girl pull out a syringe early in the episode, leaving us to assume she's a junkie. Later she does the same thing, but it's quickly confiscated by her aunt. Her aunt is the murderer, and is trying to kill the girl (who is diabetic) by locking her in a room without her insulin after she discovered a tape she used to make the (blind) victim jump out of the window, thinking there was a fire.
Inverted in the "Wormhole X-Treme!" episode of Stargate SG-1 when a bad guy pulls out a syringe to use on the good guys. The reply is "I'm hoping that's your insulin shot."
An episode of Peak Practice has a homeless diabetic who everyone assumes is the mother of an abandoned child. At one point she gets all her insulin supplies destroyed by a group of yobs.
Potential inversion twice in Cardiac Arrest reminding us that insulin is actually a very dangerous chemical. First when someone injects themselves with an overdose as a suicide and second when a deranged diabetic injects a doctor with his own insulin. The series ends with the doctor, who is the main character, being carried into the operating theater after preliminary treatment on a couch.
The wonderful episode of Seinfeld where Elaine tests positive for opium as a result of eating too many poppy-seed bagels. Meanwhile, the showerheads in Jerry and Kramer's building have been changed to a low-pressure model, and they're so desperate for a decent wash ("I feel like I have little bugs crawling all over me!") that they end up furtively buying new ones from a shady guy with a van.
Appears to be a case of Truth in Television since MythBusters proved that eating poppy seeds actually can register a positive for opium with some drug tests, in spite of the fact that there are only trace amounts in the poppy seeds used for cooking.
Another Seinfeld is entitled "The Sniffing Accountant", and you can probably guess that one from the title. (It turns out to be an allergy to mohair... until it turns out it actually is drugs).
On Webster the reason George doesn't want Webster's uncle played by Ben Vereen to have custody of him. He saw his syringe in Webster's parents' bathroom years ago and assumed he was shooting himself up with heroin. It turns out he's diabetic.
Meta example: Everyone wanted to know what was up with that syringe Inara was holding. Joss assured the public at large that it wasn't a suicide shot or some kind of narcotic (the closest to actually confirming what it was, was implying that a second season of the show would have explored some sort of illness on Inara's part).
Neighbours. After seeing Danni Stark inject insulin for her diabetes, Michael Martin spreads a rumour that she is using drugs. She goes along with it to get attention.
In the second season of Everwood Delia Brown walks in on Linda Abbott taking a lot of pills in their bathroom. She tells Linda she "thought she was a junkie" when she finds out she was actually Taking medication to maintain control of her HIV.
CSI: New York Hawkes, after his girlfriend was using pot and came over to his place. He got the residue in his system and it showed in his random NYPD drug test. It wasn't enough to get him fired, though Mac called him on it.
A similar thing happened to Delko in CSI: Miami, except it was his sister (who was taking it for medical reasons) and him testing positive (along with a few other things) convinces an IA agent that the lab's corrupt causing her to try and plant evidence to "prove" it.
On Degrassi, when Marisol is planning a trip for her and her boyfriend Mo and needs to fill out a travel insurance form, he won't let her fill it out for him. She gets paranoid and goes through his bag, finding needles. Marisol assumes that he is a junkie (despite the fact that she doesn't even know what drug uses needles) until she finds out he's diabetic.
One episode of Emergency! had a know-everything former combat medic training to become a paramedic. The patient he insisted was on an acid trip turned out to be a diabetic with dangerously low blood sugar, who would have died if Roy and Johnny hadn't given him glucose.
The Suicidal Tendencies song "Institutionalized" has its focus character get sent to an institution partly because of this trope.
In one Zits strip, Walt is suspicious when Jeremy start burning incense in his room. Thinking it is to cover up the smell of pot smoke, he bursts in to discover Jeremy is using it cover up the smell of him cooking waffles.
Played for horror in one of Hunter: The Vigil's NPC monsters. A Changeling who is constantly shivering is assumed by her coworkers to be an addict, when in fact it's because she has an icicle stuck in her heart.
One of the subplots that appears in Unisys game A Week in the Life of..., where a character notices someone else taking an insulin injection.
In Persona 4 Arena,Akihiko's tagline is "The Two-Fisted Protein Junkie" because of his insistence on a fighter's need for protein drinks. A South American bartender gets the wrong idea when Akihiko asks for a fix and mentions withdrawal.
Goes badly in Kara no Shoujo when Mizuhara Toko mistakes her friend Kuchiki's medication for drugs - and gets rid of it. This means that Kuchiki is without it when her condition begins to act up and deliriously wanders into traffic where she passes out and gets hit by a truck.
One Saturday Morning Breakfast Cereal plays with this - the situation is a woman being held back from a syringe, and we assume she's on some sort of illegal drug. Then it turns out it's insulin. Double Subversion: she's not diabetic.
In a story arc of Sandra and Woo the girl Larisa is caught by a teacher when giving herself an insulin shot during recess. The initial confusion was logical enough, since Larisa did play up the "junkie" image for a laugh, but even after being informed that she literally needs it to live, the principal continues insisting that taking this "dangerous drug" violates the school's zero tolerance policy against drugs. The plot was inspired by a similar incident on a U.S. middle school in the 1990s. Start reading here.
In Shiniez Ally's neglect of her day to day life during her relationship with Alan caused her mother to make this mistake when in reality she was addicted to the BDSM.
In an episode of The Simpsons where Marge and Homer reminisce about college days; Homer had become a grunge rock star, but due to Marge dating her teacher he had become withdrawn and depressed. He wrote a song for her (a parody of "Glycerine") and when she saw it she went to go get him back: She found him with a syringe in his arm. When she went to pull it out, Homer cried "But I need it!". Turns out it was insulin, as Homer became diabetic from drinking too many Starbucks Frappacinos, He really did need it.
She does this again with Bart when he gets into a trading card game and she mistakes him for a dealer.
In Batman Beyond Terry's mom jumps to conclusions upon finding suspicious looking patches in her son's bag. To her credit these were drugs, a steroidal compound known as "slappers," but Terry was bringing them to his boss for analysis. Terry's (entirely truthful) excuses don't help: "They're not mine! I found them!"
The South Park episode "Major Boobage" does this with cat urine. When Kyle's dad Gerald finds out, he freaks out and latches onto both the Jerkass Ball and the Villain Ball by having cats banned from South Park. For some reason, while the boys were trying to help Kenny get over his cat urine addiction, Kyle decides to take a cat home. Bad idea, because Sheila finds it in his drawer, and it leads to a conflict between Kyle and his parents, who accuse him of being a smuggler and punish him without any explanation. Shortly after, it turns out that Gerald was an addict himself, hence why he stupidly banned the cats.
Merry managed to pull this off to a transit cop in Whateley Universe while not even being caught taking anything. This was accomplished through a combination of migraines, hunger from increased mutant metabolism, and being homeless and traveling by tunnels at the time.
During the Sydney Olympics a cleaner received a needle stick injury whilst cleaning an Australian athlete's room, but the needle turned out to be for a vitamin shot.
This is a real pain in the ass for monitoring cyclists as many top athletes will inject themselves with vitamins in between races to recover faster. The equipment is nearly identical to what you would need to dope your blood or do EPO.
If somebody is having to either have a massive number of blood-draws or injections — or both — for medical reasons, they might actually have to carry around a card or other paperwork verifying that the tracks are all perfectly legitimate.
Not quite following the trope as straight as others, but pretty close: When somebody is taking medication for mental illness, many people (especially people such as Moral Guardians and The Fundamentalist) tend to think that it's as if they were taking something recreationally. This can lead to embarrassment and awkward situations.
Epileptic seizures can come across like drunkenness or drug intoxication to an untrained observer, especially because someone unfamiliar with epilepsy most likely equates "seizure" with grand mals (in which one collapses) and not with a petit mal seizure (which involve a general 'spacing out') or temporal lobe seizures (with mood swings and hallucinations).
Epileptics take a wide range of drugs and they come in both pill and injected forms.
Diabetics undergoing insulin shock often appear drunk. It's actually a medical emergency.
Similarly, many diabetics don't like checking their blood sugar (which requires a pinprick on the finger or arm) or taking insulin in public because of this trope. Many diabetics choose to do it in the restroom or another more private place to avoid stares and questions.
Can happen in some contexts with some illnesses: the more severe or stereotypical the symptoms are, the more likely someone is to be Mistaken for Junkie, even if their illness is real. Common with several pain syndromes and chronic pain disorders and ADHD - the medications needed to treat them are usually addictive and controlled substances, and the more stereotypical or "over the top" the symptoms are (or, alternately, the more subdued) the more likely the diagnosis is to be missed or dismissed.
Hyperthyroidism and hypothyroidism are both very bad for this. Someone severely hyperthyroid will sometimes appear for all intents and purposes to be on something like methamphetamine or similar because the symptoms of severe hyperthyroidism can come across very similarly to those of meth addiction. Hypothyroidism, on the other hand, can present similarly to someone drugged up on sleeping pills or opiates with its tiredness, sluggishness, and similar.
Because testosterone is a steroid, and therefore a controlled substance in many countries, FTM transgender people often have to deal with this - especially if they use injections or gel instead of implants, and are often advised to keep a copy of their prescription and/or a letter from their treating doctor.
This also led to quite nasty transphobic discrimination against FTM athletes for a long time - seeing as testosterone is, obviously, a banned substance in most organized athletics, and there were cases of female non-transitioning athletes doping with it, FTM persons (or even women mistaken for them such as Caster Semenya) were often disqualified from organized athletic competitions or otherwise forced to "prove" their female gender and that their body was naturally producing any excess of testosterone. This situation is slowly beginning to change with some sports accepting transmen who have legitimate proof they are indeed transitioning to male and whose testosterone levels are within the cisgender male range, but still lingers on in others.
The assumption that transgender (whether FTM or MTF) people are "on drugs" or "more likely to use drugs" also leads to discrimination in many other contexts: for example, some people will refuse to room with/rent property to/work with transmen or transwomen because of this assumption. In some places and some settings, laws have been written to make this discrimination illegal, but in many other places there isn't a law against discriminating against a transgender person, and even if there is a law, especially if the bigot can play it off to "I think they're on drugs," rather than outright trans hate, it's incredibly hard to do anything about it legally, because discriminating against drug users is codified into the law itself and heralded as a good way to reduce drug use.
Patients who have a medically managed addiction, even though the addiction is a side effect of treatment and not being a "junkie," due to being the Functional Addict and having a steady, managed supply, are often written off as junkies because they are addicted - never mind that addiction is a medical process as much as it is a mental one, and someone can be physically dependent on, say, pain medication to treat chronic pain while not using it for recreation.
In a non-drug variant, anytime a musician appears with powder on their clothes, they will almost automatically be assumed to be using drugs - even if they had just eaten something with powdered sugar on it or it's face powder from freshening stage makeup.
A certain brand of catnip mouse decided to package the catnip separately and give you a little extra to restuff the mouse with, in a clear plastic tube with a black twist top at the end. People then come in to your house to find a crushed, dried green plant in a suspicious looking tube on your counter. Hilarity ensues.
Certain groups of people are automatically assumed to be junkies by the majority of others, regardless of evidence or lack thereof. Prostitutes are assumed to be doing their work for the drugs, and the homeless are often assumed to take drugs since they probably up to no good anyways, at least as far as "upstanding" citizens are concerned. While these groups certainly do have members that take drugs, many do not, and the stereotyping just further isolates them from success and acceptance in society.