Using more than one drug per session is often called ‘polydrug use’ or ‘mixing drugs’. This involves using more than one drug at a time, including the use of alcohol and prescription medication. People may use different combinations for various reasons, in some cases they might mix drugs for combined effects. In some situations, people may be on medication which are prescribed by a doctor and do not realise that it can negatively react with alcohol or other drugs. 

Anything you consume can have some form of interaction; however, some drug combinations may be more risky than others. It is always safer not to use drugs at all and each person will react differently based on personal factors (physical and mental health, if they have underlying conditions), the contents and potency of the drug they are using and the setting they are in (if they are with people they trust, if they are in a new setting, if it is a loud environment like a festival etc).