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The term drug overdose (or simply overdose or OD) describes the ingestion or application of a drug or other substance in quantities greater than are recommended or generally practiced. An overdose is widely considered harmful and dangerous as it can result in death.
The word "overdose" implies that there is a common safe dosage and usage for the drug; therefore, the term is commonly only applied to drugs, not poison, though it should be noted that even certain poisons are harmless at a low enough dosage. Drug overdoses are sometimes caused intentionally to commit suicide or as self-harm, but many drug overdoses are accidental and are usually the result of either irresponsible behavior or the misreading of product labels. Drug overdose often happens as a result of the use of multiple drugs with counter indications simultaneously (for instance, heroin/certain prescription pain medications and cocaine/amphetamines/alcohol.) Usage of illicit drugs that are of unexpected purity, in large quantities, or after a period of abstinence can also induce overdose. Cocaine users that inject intravenously can overdose accidentally as the margin between an optimal flash and an overdose is small.
Accidental overdoses can eventuate out of a number of different causes including overprescription, failing to recognise a mdrug's active ingredient, or unwitting ingestion by children A common unintentional overdose in young children involves multi-vitamins containing iron. Iron is a component of the hemoglobin molecule in blood, used to transport oxygen to living cells. When taken in small amounts, iron allows the body to replenish hemoglobin, but in large amounts it causes severe pH imbalances in the body. If this overdose is not treated with chelation therapy, it can lead to death or permanent coma.
Signs and symptoms of an overdose varies depending on the drug or toxin exposure. The symptoms can often be divided into differing toxidromes. This can help one determine what class of drug or toxin is causing the difficulties.
Diagnosis of an overdosed patient is generally straightforward if the drug is known. However, it can be very difficult if the patients cannot (or refuse to) state what drug they have overdosed on. At times, certain symptoms and signs exhibited by the patient, or blood tests, can reveal the drug in question. Even without knowing the drug, most patients can be treated with general supportive measures.
In some instances, antidotes may be administered if there is sufficient indication that the patient has overdosed on a particular type of medication.
Stabilization of the ABCs are the initial treatment of an overdose. This involves establishing a stable airway, breathing rate and circulatory system as an essential first step. Ventilation is considered when there is a low respiratory rate or when blood gases show the person to be hypoxic. The next necessary step is to treat for shock. Investigations should be carried out in labs to help identify the drug(s) at hand such as glucose, urea and electrolytes, paracetamol levels and salicylate levels. Monitoring of the patient should continue before and throughout the treatment process, with particular attention to temperature, pulse, respiratory rate, blood pressure, urine output, electrocardiography (ECG) and O2 saturation.
Specific antidotes are available for certain causative agents. The overdose agent is usually determined either via history or laboratory toxicology.
Negative drug-drug interactions have sometimes been misdiagnosed as an acute drug overdose, occasionally leading to the assumption of suicide. 
Additionally, recent psychological research indicates that "overdose" may be, in many cases, a misnomer. Most deaths attributed to heroin overdose, for example, are not technically due to "overdose"of heroin they have taken many times before. There is compelling evidence that taking heroin in a new or different environment than usual may lead to overdose. In the terms of Pavlovian conditioning, the environment where the addict usually takes the drug (for example, if he always injects in the same room with the same people) serves as the conditioned stimulus, while the drug effect of heroin serves as the unconditioned stimulus. The body tends to try to maintain homeostasis, so it creates a compensatory response to counteract the effects of the drug. In the case of heroin, which decreases pain sensitivity and slows breathing, the body's compensatory response would be to increase pain sensitivity and speed up breathing. As the environment (CS) and drug effect (US) are paired over and over, the environment alone becomes sufficient to evoke the body's compensatory response to heroin. This compensatory response, triggered by the environmental cues alone, is the conditioned response. As Pavlov's dogs learned to salivate at the ring of a bell because the bell was often paired with food, a heroin user's body creates a chemical, opposing response to heroin when the proper environmental cues are present. For this reason, the heroin abuser becomes able to take larger and larger doses of the drug, because his body creates a stronger and stronger compensatory response to its effects. "Overdose" often (more than half the time) occurs when the heroin abuser injects in a new environment. In this case, the environmental cues are not present, so the body does not produce the compensatory response required to make the usual large dose of heroin tolerable. The result is often death.
Combined Drug Intoxication or Multiple Drug Intake, is mistakenly reported by news media as Drug overdose, but it is not the same. CDI does not require drug overdose to kill a person. Death is caused by the simultaneous use of too many drugs.
One of the most common drugs to be implicated in Combined Drug Intoxication deaths, or in non-lethal overdoses causing harm to the body is acetaminophen (or Paracetamol as it is known in some countries), an analgesic that is available over the counter. While considered harmless and beneficial when taken at recommended dosages, acetaminophen can be acutely toxic to the liver when taken in amounts exceeding its recommended dosage; this toxicity is compounded when the drug is taken in combination with alcoholic beverages, especially by chronic drinkers and people with preexisting liver disease such as hepatitis. In addition, long-term use of acetaminophen at high dosage (and especially concurrent with alcohol) is a common cause of chronic damage to the liver. See the main article on paracetamol toxicity for more information.
Since Paracetamol is not considered, by itself, an addictive medication, complications arising from its overuse are often referred to in medical literature as Paracetamol Poisoning. However, since paracetamol is often contained in formulations which contain other drugs with a high potential for abuse, it is often ingested in amounts far exceeding its therapeutic dose in order to get a "high" from the coexisting drug. Examples include over-the-counter cough syrup and cold remedies which include, along with paracetamol, dextromethorphan, a cough suppressant that, when taken in high amounts, can cause hallucinations and euphoria. Because of its availability as an over-the-counter preparation in most jurisdictions (in brands such as Robitussin), dextromethorphan abuse is particularly popular among teenagers because it is easy to obtain. In addition to paracetamol, other medications like Guaifenesin, and pseudoephedrine are often present in dextromethorphan formulations, although many preparations contain dextromethorphan by itself, without any other drugs. It is common for a user to ingest an entire bottle of dextromethorphan-containing syrup to obtain the amount needed to get the wanted effects, which can deliver a dose of paracetamol (when present in the formulation) that is well above levels that can cause acute toxicity. Refer to the main article on non-medical use of dextromethorphan for more information.
Paracetamol is also combined with many narcotic analgesics that are, in most countries, strictly regulated as controlled substances because of their highly abusive potential. When patients or recreational users of these medications (examples include the brand names Opana, Dilaudid, and Oxycontin) ingest these drug combinations in large amounts, they risk acute paracetamol poisoning (and sometimes overdose of the desired narcotic) or, over time, chronic hepatic damage. The abuse of these medications is on the increase, despite the fact that most physicians who prescribe them supply the patient with only a limited quantity in order to prevent their potential for chronic use and abuse. However, it is relatively simple for a user to find supplies of these medications, either on the street or by "jumping" from one physician to the other; dentists are often approached for prescriptions, since narcotic medications are given quite frequently to patients with dental pain, due to their efficacy in pain management for many dental problems. However, most dentists give prescriptions for a very limited supply of a narcotic analgesic, because, with treatment, most cases of tooth pain are relatively short-lived. NSDEA
While they do not give separate figures for drug overdoses and other kinds of accidental poisoning, the National Center for Health Statistics report that 19,250 people died of accidental poisoning in the U.S. in the year 2004 (8 deaths per 100,000 population).
In 2008 testimony before a Senate subcommittee, Medical Epidemiologist Dr. Leonard J. Paulozzi of the Centers for Disease Control and Prevention stated that in 2005 (the most recent year for which data was available) more than 22,000 American lives were lost due to overdoses, and the number is growing rapidly. Dr. Paulozzi also testified that all available evidence suggests that unintentional overdose deaths are related to the increasing use of prescription drugs, especially opioid painkillers.