Polydrug use is when a person uses more than one type of drug, either at the same time or one after another in the same session.
Polydrug use occurs when a person:
uses two or more drugs in combination
uses one drug to counteract the effects (or the after-effects) of another
uses different drugs at different times over a short period
Polydrug use can multiply the risks of individual substances. For example, combining depressants: Consuming GHB and alcohol together massively increases the risk of both.
Fatal overdose as the depressants work together to slow down both the heart and breathing rate and nonfatal overdose, which can result in permanent brain damage.
GHB & alcohol are particularly dangerous to combine as they use the same metabolic pathway. This means GHB can build up in your liver while the alcohol is processed, and then be released all at once in a dangerous overdose.
Other examples are using two stimulants together or using a stimulant and a depressant together.
A fantastic resource on drug combinations is combo.tripsit.me.
As a general rule dangerous combinations include:
Depressants with more depressants
Stimulants with more stimulants
Depressants & stimulants