Definition & clinical relevance

“Other” or “unknown” substance use disorder is a working category used when clinical evidence indicates problematic use (craving, loss of control, continued use despite harm) but the precise agent cannot be established at presentation. Management relies on symptom clusters, risk assessment and linkage to appropriate services.

Why identity may be unknown

• Poly-substance use, new psychoactive substances (NPS) with variable detection, or absence of witnesses.
• Stigma and patient reluctance to disclose.
• Unlabelled or counterfeit substances sold in informal markets.

Identification & screening

• Use standard screening tools: AUDIT-C (alcohol), CAGE, ASSIST, and the single-question drug screen — “In the past year, how often have you used an illegal drug or used a prescription medication for non-medical reasons?”
• Assess severity: DSM-5 criteria for Substance Use Disorder (mild/moderate/severe) based on behaviour and functional impact.
• Look for red flags: overdose history, withdrawal symptoms, injection marks, social/occupational decline, criminal/legal problems.

Clinical assessment — core components

1. Detailed psychosocial history: timeline, frequency, route of use, last use, prior treatments, support systems.
2. Physical exam: signs of injection use, skin infections, liver disease stigmata; neurological exam for tremor, ataxia.
3. Mental state exam: suicidality, psychosis, cognitive impairment.
4. Medication review and interactions (prescribed opioids, benzodiazepines, psychotropics).
5. Collateral information and pharmacy/clinic records where possible.

Immediate management & risk mitigation

• Stabilise acute medical issues: overdose, withdrawal, infections, injuries.
• If recent opioid exposure suspected, consider naloxone training and take-home naloxone on discharge.
• For possible alcohol or benzodiazepine dependence, use symptom-triggered withdrawal protocols (CIWA‑Ar) or fixed-dose benzodiazepine tapers; provide thiamine.
• Address acute psychiatric risk (suicidality, severe psychosis) with safety planning and urgent psychiatry input.

Harm reduction strategies

• Needle/syringe programs and safe injecting advice to reduce BBV transmission and SSTIs.
• Provide naloxone and teach peers/family how to use it.
• Encourage safer use practices: avoid mixing depressants, use with a sober companion, start low-go-slow with unknown potency.
• Vaccination for hepatitis A/B and HIV testing and linkage to care where relevant.

Treatment pathways — tailoring to probable class

Even when the exact substance is unknown, many evidence-based treatments are class-specific. Use clinical features to infer likely class (opioid-like, stimulant-like, sedative-like) and start appropriate interventions while investigations continue.

Opioid use disorder (probable)

• Offer opioid agonist treatment (methadone, buprenorphine) when indicated and discuss maintenance vs detoxification options.
• Link to needle/syringe programs, counselling and psychosocial supports.

Stimulant use disorder (probable)

• Psychosocial interventions: contingency management, CBT, and community-based programs show the best evidence.
• No approved pharmacotherapy for stimulant use disorder—treat comorbidities and support harm reduction.

Alcohol / sedative-hypnotic (probable)

• Manage withdrawal with benzodiazepine protocols; consider long-term psychosocial interventions (CBT, mutual-help groups).
• Discuss pharmacotherapy for relapse prevention (naltrexone, acamprosate, disulfiram) if alcohol dependence confirmed.

Psychosocial treatments & rehabilitation

• Motivational Interviewing to resolve ambivalence and enhance readiness for change.
• Cognitive-behavioural therapy for relapse prevention and coping skills.
• Contingency management where available (incentives for abstinence or engagement).
• Family therapy and peer support groups (NA, SMART Recovery) to build social support.

When to involve specialists

• Complex dependence, co-occurring severe mental illness, pregnancy, youth or older adults — refer to specialist addiction services.
• Overdose requiring ICU care or unusual toxidrome — engage toxicology and critical care teams.
• Consider medico-legal or forensic liaison for patients with legal issues or mandated treatment.

Key takeaways

• Treat patients, not labels: use clinical features and risk profile to guide immediate care when substance identity is unknown.
• Harm reduction (naloxone, needle/syringe programs) and linkage to addiction services save lives and improve engagement.
• Use evidence-based psychosocial and pharmacological treatments tailored to probable substance class and patient preference.
• Address social determinants and involve multidisciplinary teams for long-term recovery planning.