Naloxone (UK version of Narcan) come in the form of a syringe & is used to reverse the effects of someone having a opioid overdose. If you or are around people that use opioids, it is highly recommended to carry naloxone with you. It causes no real harm to those injected with it and it may save a life.
Naloxone is easily available in the UK; best way to attain it is to request some from a public drug care service or speak to a local medical professional for more information.