First line care for a bad psychedelic trip is non-pharmacological: a calm, safe setting, reassurance, and clinical monitoring when needed. If symptoms remain severe, some medications are discussed in the literature as potential options in supervised medical settings, but evidence is limited.
A recent review highlights selective 5-HT2A antagonists as the most targeted candidates, particularly ketanserin where available; pirenperone is also noted but availability and side effects are concerns. If ketanserin is not accessible, the authors consider risperidone or paliperidone as practical choices in clinical contexts.
For anxiety and agitation, benzodiazepines can help calm and sedate but may not terminate the psychedelic state. Other proposed options include cyproheptadine, trazodone, and mirtazapine; haloperidol may be less suitable and could worsen anxiety. Alcohol and cannabis are not recommended.
These points come from a critical review that proposes a provisional framework rather than proven guidance, and it calls for more research; see the summary on PubMed here: review of candidate medications.
If in doubt, seek urgent medical help.