Dissociative NPS are a class of hallucinogenic drugs, which distort perceptions of sight and sound and produce feelings of detachment – dissociation – from the environment and self. They produce hallucinogenic effects, which may include sensory
deprivation, dissociation, hallucinations, and dream-like states or trances. These NPS act like Ketamine and nitrous oxide
Examples are PCP, MXM and 3/4MeO-PCP.

Short term risks include panicked state, unconciousness, susceptibility to accidents from uncoordination in perception of body and time, severe confusion and disorganised thinking, nausea and vomiting, depersonalisation
Long term risks include dependancy, permanent mental health problems, incontinence. Not all long term effects are currently known

Dissociatives are commenly swallowed, snorted or inhaled.

For more (fairly technical) information on the different dissociatives, you can visit this Wikipedia page. Be aware though, that this page is not scientifically checked.

Source: 1, 2, 3