Overdose

Depressants

Depressants, such as alcohol and benzodiazepines (e.g. Xanax and Valium), can be incredibly dangerous when taken in excessive doses, and especially when mixed together. Having too much alcohol can cause severe complications which include: asphyxiation, dehydration, irregular heartbeat, and in severe cases, death. Similarly, benzodiazepines can induce a coma, reduced ability to breathe, and even heart attacks in some situations.

Dissociatives

Dissociative anaesthetics such as ketamine are reasonably well tolerated at low doses. However, they can cause changes in heart rate and blood pressure which can be dangerous. DXM can also cause greatly reduced or even complete cessation of breathing, and disturbing psychological effects.

Opioids

Opioids, such as heroin and fentanyl, can very easily, and often accidentally, be taken in quantities that cause an overdose. Like depressants, opioids can cause respiratory depression, coma and death. With very strong opioids like fentanyl, which is approximately 100 times more potent than liquid morphine, overdose can occur with extremely small amounts.

Overdoses on fentanyl have even occurred unknowingly, as small quantities are sometimes accidentally pressed into fake Xanax pills. This had fatal consequences for the individuals who took the tainted pills. Only a tiny dose of fentanyl is required to cause death. As little as 2mg.

Psychedelics

Psychedelics such as LSD or psilocybin can have very unpleasant side effects. These include: anxiety, vomiting, and hyperthermia. They've also been known to trigger onset of acute psychosis in people with a family history of schizophrenia. Unlike drugs like fentanyl and heroin, psychedelics have rarely been known to directly cause death themselves. Deaths as a result of psychedelic ingestion are typically the result of risky behaviour while on the drug.

Stimulants

Overdoses on stimulants such as amphetamine or cocaine put the cardiovascular system at great risk. They can easily cause heart attacks, dangerously high blood pressure, cerebral haemorrhage (bleeding in the brain), as well as involuntary movements and other hazardous symptoms. However, there is also a risk of developing psychosis, which is especially prevalent with amphetamine and methamphetamine.

Empathogens, such as MDMA (ecstasy), can cause serotonin syndrome when taken in either, excessive quantities or when taken alongside certain antidepressants, such as MAOIs. There is also a risk of overheating and dehydration. However, the biggest risk associated with MDMA is pills cut with other dangerous chemicals, which have been known to kill unsuspecting clubbers.

References: Fatal alcohol and benzo poisoning; Serotonin syndrome; MAOIs; Volteface; Fentanyl lethal dose; Amphetamine induced psychosis; Ketamine and PCP; Drug induced psychosis