Mistaken for Junkie

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 drugs. Sometimes Mistaken for Junkie is used to criticize the Drugs Are Bad hysteria.

Fanfic

Parodied by Foxx Fireart in his/her Code Geass fan-comics. In R2 when Lelouch is going to use drugs to escape from his depression

Film

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.

Literature

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.
Briefly done in Dan Brown's Digital Fortress, where David Becker mistakes a Damsel in Distress at the airport for a drug addict.

Live Action TV

Happened once in Home and Away, with the insulin shot.
- Happened again more recently, with pills this time around. It's understandable, however, as Belle actually was a junkie for the first half of 2009.
A variant with Estrogen shots in the BBC drama Family Man.
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 stepped up to heroin but he had actually stepped down to methadone.
- This trope is what actually attributed to the loss of House's leg. 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, but it's quickly confiscated by her aunt. Her aunt is crazy, and the killer of the week, and is trying to kill the girl (who is presumably a diabetic) by locking her in a room without her insulin after she discovered details about the aunt's previous murder.
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.
- And Kramer buys the one designed for elephants.
- 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).
In That '70s Show Eric's parents thought that he's on drugs when he started acting weird; actually, he saw them having sex.
When Dexter is caught in his web of lies at one point and thinks he is going to be exposed as a serial killer, instead it is assumed that he is a drug addict. He goes along with it.
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: NY 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.

Newspaper Comics

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.

Tabletop Games

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.

Video Games

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.

Web Comics

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. 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. She's then sent to the principal who informs her 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.

Western Animation

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 introverted 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.
- She does this again with Bart when he gets into a trading card game and she mistakes him as 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!"

Web Original

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.

Real Life

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 mind-altering. This can lead to embarrassment and awkward situations.