According to the United Nations Office on Drugs and Crime (UNODC), it is estimated that 1 in 20 adults, or a quarter of a billion people between the ages of 15 and 64 years, used at least one drug in 2014. Cannabis remains the most commonly used drug at the global level, while amfetamines remain the second most commonly used drug. Although the use of opioids and prescription opioids is less common, opioids remain major drugs of potential harm and health consequences. A sharp increase in heroin use in some markets where it was previously declining shows that heroin remains one of the major drugs of public health concern. Many people who use drugs, both occasionally and regularly, tend to be polydrug users who use more than one substance concurrently or sequentially. 
An opioid is a synthetic or natural agent that stimulates opioid receptors and produces opium-like effects. Opiates are opioids that are naturally derived from the opium poppy (e.g., morphine). They are used to treat pain but may also be abused because of their euphoric effects. Opioid use disorder is defined as a problematic pattern of opioid use, leading to clinically significant impairment or distress, occurring in a 12-month period,  and may include symptoms of tolerance and withdrawal, despite knowledge that continued opioid use is the cause of these problems. Commonly abused opioids include codeine, fentanyl, heroin, morphine, opium, oxycodone, and hydrocodone.
Overdose occurs when the quantities taken are larger than can be physically tolerated, resulting in central nervous system (CNS) and respiratory depression, miosis, and apnoea. Can be fatal if not treated rapidly.
Cocaine is a drug of abuse that is usually either insufflated (snorted), injected, or smoked in its freebase form (crack). Cocaine is a type IA antidysrhythmic, local anaesthetic, and sympathomimetic. Episodic cocaine use leads to short-lived states of autonomic arousal. Chronic use can lead to scarring of heart tissue and myocardial hypertrophy, with increased myocardial calcium content resulting in an increased risk for lethal arrhythmias and sudden death. Cocaine use causes a hyperadrenergic state associated with abnormal mentation. The symptoms of any hyperadrenergic state include nausea, jitteriness, trouble concentrating, anxiety, paranoia, and euphoria.
Refers to adverse events that occur within minutes or hours of excessive cocaine use. These events, which can occur in combination or isolation, include hyperthermia, rhabdomyolysis, dysrhythmia, ischaemia, intracranial haemorrhage, agitation, psychosis, and seizures.
Patients may die suddenly before treatment can be given.
Involves use of a class of non-catechol sympathomimetic amines, including amfetamines, methamfetamines, and methylenedioxymethamfetamine (MDMA, ecstasy). These are taken by oral and intravenous routes, by nasal insufflation (snorting), and by inhalation (smoking), resulting in either acute or chronic toxicity. Patients have a high probability of repeated abuse of amfetamines after documented acute or chronic abuse.
Global cannabis consumption has remained somewhat stable in recent years.  Acute health effects include impairment of cognitive development (children and adolescents) and psychomotor performance. Chronic health effects include further cognitive impairment, dependence, exacerbation of schizophrenia, and airway and lung damage associated with smoking the drug.
The deliberate inhalation of a volatile substance to achieve an altered mental state.  Inhalants used include volatile solvents from household and industrial products, aerosol propellants, gases from household, industrial, and medical products, and nitrites. Hypoxia and heart failure can occur within minutes. Longer-term adverse effects include hearing loss, peripheral neuropathies, and liver and kidney damage.
Includes LSD, peyote cactus, psilocybin (from certain types of mushrooms), and PCP (phencyclidine). Psychological effects can be unpredictable.
Overdose can be intentional in suicidal patients, accidental in combination with other CNS depressants such as alcohol and opioids, and in older people (who commonly have diminished drug clearance and polypharmacy), and occasionally by medication error. The key feature is excessive sedation with unremarkable vital signs and anterograde amnesia. Larger doses can cause coma and respiratory depression. Acute management consists of maintaining airway, respiration, and haemodynamic support while excluding other diagnoses. Assisted ventilation may be necessary. Death is uncommon.
Tricyclic antidepressants have a narrow therapeutic index and therefore become potent cardiovascular and CNS toxins in moderate doses. Best markers for suspected overdose are a history of depression, suicidality, and overdose, with a sudden deterioration in mental status and vital signs.
Testosterone derivatives are used to improve athletic performance or to increase lean body mass and muscle size. Adverse effects in men include acne, oily skin, disproportionate muscular development of the upper torso, changes in libido, testicular atrophy, scrotal pain, impotence, infertility, temporal hairline recession, irreversible gynaecomastia, and increased voice pitch. In women, adverse effects include acne, oily skin, muscular development of the upper torso, menstrual irregularities, and changes in libido. The potential irreversible masculinising effects include hirsutism, male pattern baldness, deepening of the voice, and clitoral hypertrophy.
Occurs by acute single ingestion of a large amount or by repeated ingestion of an amount exceeding the recommended dosage or by multiple doses. Poisoning may cause various degrees of liver injury including fulminant hepatic failure and hepatorenal syndrome. Patients are often asymptomatic or have only mild gastrointestinal symptoms at initial presentation. Initial presentation with coma and severe metabolic acidosis is rare.
Alcohol dependence is a chronic, relapsing disorder that results from a variety of genetic, psychosocial, and environmental factors.  It is characterised by increased tolerance to the effects of alcohol, the presence of characteristic withdrawal signs and symptoms, and impaired control over the quantity and frequency of drinking.  Prolonged exposure causes adaptive changes in the brain receptors and neurotransmitters, which are responsible for various effects such as addiction, tolerance, and withdrawal. Alcohol dependence, particularly when chronic and severe, can be associated with a variety of medical and psychiatric sequelae.
In the US, cigarette smoking prevalence has declined from 20.9% in 2005 to 15.5% in 2016.  Smoking is associated with heart disease, multiple types of cancer, and COPD. Physicians and other healthcare professionals should play a central role in motivating and assisting patients who smoke to quit. 
Children may ingest a toxic substance accidentally while exploring their environment, or deliberately in response to stress or underlying mental problems, or in an attempt to get "high". Agents consumed may be pharmaceutical substances; drugs of abuse (including alcohol); toxic plants, berries, or mushrooms; or chemicals. Diagnosis is based on a combination of thorough clinical assessment and comprehensive laboratory investigation to identify all ingested substances.
Acute coronary syndrome refers to acute myocardial ischemia caused by atherosclerotic coronary disease, and includes ST elevation myocardial infarction (STEMI), non-STEMI, and unstable angina. Cocaine accounts for up to 25% of acute MIs in people aged 18 to 45 years.  The lifetime risk of non-fatal MI with cocaine use is 7 times the risk in non-users. In the hour after cocaine is used, the risk of MI is 24 times the baseline risk. It is probably due to cocaine-induced coronary vasospasm and thrombosis, in addition to a direct effect on heart rate and arterial pressure. Cocaine also has direct myocardial toxic properties. 
The term STDs includes a range of clinical syndromes that can be acquired and transmitted through sexual activity and may be caused by various types of pathogen, including bacteria, fungi, viruses, and parasites.  Drug users are at risk from STDs from both injection of drugs and taking part in high-risk sexual behaviour under the influence of drugs.
Needle-sharing injection drug use with an infected source is a risk factor for HIV infection.
The pathological end-stage of any chronic liver disease. It most commonly results from chronic hepatitis C and B (which can be caught from intravenous drug use), alcohol misuse, and non-alcoholic fatty liver disease. The main complications of cirrhosis are related to the development of liver insufficiency and portal hypertension and include ascites, variceal haemorrhage, jaundice, portosystemic encephalopathy, hepatorenal and hepatopulmonary syndromes, and the development of hepatocellular carcinoma.
Depressed patients may abuse drugs to 'dull the pain' or to address feelings of low self-worth. Additionally, the chemical effects of drug use may cause depressed mood.
Drug use is very common in patients with schizophrenia.  Relates to an increased incidence of psychosis and psychotic decompensation.
Use of sympathomimetic street drugs (e.g., cocaine, LSD, amfetamines, ecstasy) predisposes to hypertensive emergency.
Chronic intravenous drug abuse leads to scarred and/or collapsed veins and skin changes that can be a hallmark of heroin addicts. These skin changes may develop into infections such as cellulitis and abscesses that require medical care with antibiotics and possibly surgery.
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This overview has been compiled using the information in existing sub-topics.
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