Chemsex is sex in combination with taking drugs in order to enhance the sexual experience.
Chemsex (or kemsex as you sometimes say in Swedish), just like the use of drugs in large, is more common abroad. In destinations where many gays like to travel on holidays such as London, Berlin, Paris etc. it is not uncommon to encounter this phenomenon. It is sometimes something that is openly requested by clubs or dating apps.
No matter how exciting it may seem to have sex for many hours, perhaps with a whole bunch of guys at a party – is the same problem that can arise in the short or longer term as with drugs in other contexts. Several preparations can quickly give an addiction, side effects, difficulties in predicting how the substance will affect, dosage, depression – the list is long and not as exciting to read. The loss of control and the overconfidence in moments of intoxication can have consequences and there has been a link at group level to more STIs and HIV infections. PrEP may be important to consider in order to provide protection against HIV.
Sex experts have compiled facts about some common drugs that you can read about here (swedish).