<unk>  refers to the<unk>  two or more psychoactive<unk> s in combination to achieve a particular effect. In many cases, this makes the experience far more enjoyable than taking the substances alone.
There are many common<unk>  combinations, including, but not limited to:
alprazolam and cannabis
cannabis and alcohol
cannabis and coffee  (known as "hippie speedball")
LSD and DMT,
LSD and ecstasy (known as "candy flipping"),
LSD and magic mushrooms,
Psychedelic mushrooms and ecstasy (known as "hippie flipping" or "flower flipping"),
benzodiazepines and opioids / opiates,
benzodiazepines and alcohol,
cocaine and caffeine,
cocaine and heroin (known as a "speedball"),
cocaine and alcohol (which creates a third, more toxic chemical in the body called Cocaethylene),
cocaine and cannabis (known as "primo"),
cocaine and ketamine, (known as CK1)
ecstasy and ketamine (known as "kitty flipping"),
ecstasy and phencyclidine (known as "elephant flipping"),
ecstasy and 2C-B,
ecstasy and mescaline (known as "love flipping"),
ecstasy and GHB,
ecstasy and methylcyclohexanol,
ecstasy and Viagra (Hammerheading, very dangerous)
ecstacy and nitrous oxide
dextromethorphan and marijuana
dextromethorphan and LSD (known as "LSDXM"),
DMT and nitrous oxide,
heroin and diphenhydramine (known as "cheese")
heroin and cannabis,
nitrous oxide and cannabis
Methamphetamine, Cocaine, Ecstasy, and Ketamine (Crushed up together and snorted, known as "Trail Mix")
LSD and nitrous oxide (known as "Gasid")<unk>  often carries with it more risk than<unk>  of a single<unk> , due to an increase in side effects, and<unk>  synergy.
The phenomenon is the subject of established academic literature (e.g., Scholey AB, Parrott AC, Buchanan T, Heffernan TM, Ling J, Rodgers J (2004). "Increased intensity of Ecstasy and<unk>  in the more experienced Ecstasy/MDMA<unk> rs: a WWW study." Addictive Behaviors, 29, 743-752).
Poly drug use
Old age
Binge drinking
Hepatitis B risk factors
Smoking risk factors
Hepatitis C risk factors
Influenza risk factors
Metabolic syndrome risk factors