Mia Wallace overdoses because she uses the heroin she finds in Vincent Vega's coat, thinking it is cocaine. The film had already shown Vincent buying that drug from Lance, so the viewer can recognize the danger when the bag changes hands. Mia does not know about that purchase and acts on incorrect information about what she has found.
The mistake triggers the emergency in the second half of their night together. Vincent had come to keep his boss's wife company and was worried about the limits of that relationship; when he returns from the bathroom, he has to react to her condition. The story presents a serious overdose that Mia survives. It also does not offer a chemical analysis of the substance: Lance's praise of his product belongs to a seller's pitch. The identity of the drug and the assessment of its supposed quality come from different clues and should be treated separately.
The immediate cause is understood by following an object: the bag Vincent keeps in his coat. The drug comes from a purchase made before his visit to Mia's house. She does not witness that transaction and also does not receive a warning from Vincent when she finds the package. The viewer therefore has a prior detail that the character does not.
How the bag ends up in Mia's hands
The conversation with Lance includes a packaging detail: the seller has run out of balloons and asks whether he can deliver the purchase in a small bag. Vincent agrees. That choice becomes part of an apparently routine transaction and prepares the object that will reappear at Mia's house. That role in the story does not make one kind of wrapping a reliable way to identify drugs.
After returning from dinner, Mia is wearing Vincent's coat while he is in the bathroom. The film's final continuity makes the action clear: while looking for a lighter in the pocket and putting it back, he takes out the bag. The discovery comes from that everyday gesture. The scene does not show her searching the coat because she already knows Vincent bought heroin.
Mia takes the powder for cocaine and snorts it. The film establishes that confusion through her behavior and the contrast with the purchase we have already seen. It does not need her to say it out loud before she collapses. The lack of a conversation between them leaves the mistake uncorrected during the seconds when it could still have been recognized.
The coat belongs to Vincent, but the substance is within Mia's reach while he is away. This movement of the object explains the causal link without turning it into an intention to poison. Nor is there a scene in which Lance deliberately swaps one drug for another to deceive Mia: he sells Vincent what Vincent came to buy, long before the accident.
What explains the difference between heroin and cocaine
The U.S. Centers for Disease Control and Prevention describe heroin as an opioid. Its Heroin fact sheet, updated on June 9, 2025, notes that an overdose can cause slow, shallow breathing, coma, and death. That effect on breathing helps explain the severity of the mistake dramatized by the film, without turning its images into a full clinical diagnosis.
Cocaine belongs to another category: it is a central nervous system stimulant. The CDC's Spanish-language page on stimulants, updated on January 7, 2026, explains that it can also cause cardiovascular complications and overdose. The fact that Mia expects to use cocaine does not mean that expectation was safe; it means she does not know the substance she has just taken.
Lance presents his product as top quality, but the story does not provide a verified purity percentage. It also does not establish a medical history for Mia that would allow her opioid tolerance to be calculated. Attributing the outcome to a specific number, a particular adulterant, or an unmentioned medical condition would add an explanation the film does not support.
The narrative answer can be stated more precisely: Mia mistakes Vincent's heroin for another drug and suffers an overdose. The scene identifies the substance through its prior path. The visible signs communicate an emergency, but they do not on their own allow us to reconstruct every medical factor that would be involved in a real situation.
The problem Vincent was not anticipating
While Mia finds the bag, Vincent Vega is talking to himself in the bathroom about how to end the evening. He is trying to hold back his attraction and avoid betraying Marsellus. That concern directs his attention toward a decision he thinks he controls, while an object he has left out of sight creates a different danger.
When he returns to the room, he can no longer solve the night by leaving politely. Mia Wallace's overdose forces him to seek help and brings him back to Lance. The structure thus links the initial purchase with its consequences without inserting a flashback revelation about what was in the bag. The viewer can recognize the connection before Vincent discovers Mia.
Mia survives the episode and they agree to keep it from Marsellus. That ending makes it possible to be precise about the term used to describe the accident: it was a life-threatening overdose, not the character's death. Their relationship afterward is marked by the shared secret, although neither of them offers a full reflection on the decisions that led to the confusion.
A different question is how much Mia knows about Vincent's usual drug use. The film lets her know his job and talk with him during dinner, but it does not show that she knows about the purchase he made that afternoon. Sharing an evening does not mean sharing every piece of background information the editing has given the viewer. That difference in information is enough for a familiar object to take on a different meaning when it changes hands.
Nor is it necessary to attribute a desire to die to Mia in order to explain what happens. Her behavior is presented as use based on mistaken identification, and the emergency arrives as an unexpected consequence. Reading the accident as a deliberate attempt would alter the episode's causality and require additional clues. The distinction between what someone intends to do and the result of that action makes it possible to talk about both characters' responsibility without inventing thoughts the scene does not express or a medical history the script does not provide.