Cannabis Use in Psychotic Patients

Method

All acute admissions during the month of August 1994, aged 16–65 years, to Tokanui Hospital, Te Awamutu, were screened for psychosis by the admitting doctor using a brief checklist. Those scoring positive on at least one of the six items (delusions, hallucinations, catatonia/bizarre behaviour, marked loss of contact with reality, lack of insight, perplexity) were interviewed by SS within 48 h of admission and rated on the Brief Psychiatric Rating Scale (BPRS) [22]. Formal diagnoses were not recorded as it was felt that they might be affected by cannabis use. Patients' lifetime and recent use of cannabis and other substances was recorded in a brief questionnaire. A urine sample was taken, within 48 h of admission, for laboratory drug analysis. The immunoassay detected cannabinoid concentrations of 35 ng/mL and above, although false negative and positive results may occur. As urinary cannabinoids are present for up to 2 weeks after cannabis use, this may lead to false positive results. Interviewing was blind to urine test results.

Ethical approval had been given by Health Waikato Ltd Ethics Research Committee.

Methods

Thirty-nine patients suffering from psychotic illness were admitted during the 1-month period. Two patients declined to take part in the study and one was transferred to another hospital before he could be interviewed. Urine drug screens were completed on 35 (97%) of the remaining 36 patients. Eleven (31%) were positive for delta-9-tetrahydrocannabinol (THC). One of the patients who was urine THC-positive had another substance detected in their urine which was attributed to a prescribed cough linctus containing codeine. Of the 24 THC-negative patients, one had urinary opiates present but was also prescribed codeine cough linctus, and another had benzodiazepine present.

A comparison of the THC-positive group with the THC-negative group showed no significant differences in demographic data (p < 0.05, Chi-squared test) (Table 1). A greater proportion of the patients with a positive urine screen had a forensic history but this did not reach statistical significance. Overall, 30 (86%) of patients said they had tried cannabis at least once (Table 2). Urine THC-positive patients were more likely to report using cannabis most days, have used more potent preparations of cannabis at some time and have used other illicit substances. On the BPRS, the THC-positive group showed less conceptual disorganisation, less suspiciousness and less unusual thought content than the THC-negative group (Table 3).

Discussion

The finding that 86% of the sample reported having used cannabis at least once in their lifetime is double the figure that Black and Casswell [21] found in the general population, and in excess of their highest using age group (65% in 20–24-year-old men). This suggests that this group of psychiatric patients is more likely to use cannabis than the general population, although acute admissions may not represent all psychiatric patients. Thirty-one percent of patients were positive for cannabinols on urine testing which is a similar figure to that found in the Gambia [10], but lower than South Africa [9].

The THC-positive group showed significantly less conceptual disorganisation (thought disorder), suspiciousness and unusual thought content (delusions) than the THC-negative group. Less thought disorder [8] and less incoherence of speech [9] have been found before. However, the finding of marked hypomanic features or agitation in the cannabis group, described by these authors, was not replicated in this study. Cannabis may be acting to modify the presentation of psychosis or may produce a distinct illness. The small sample size means that associations with drug use may be missed (type II errors). Other researchers [11],[14] have found no cluster of presenting symptoms to be related to cannabis use.

Conclusion

The majority of psychotic patients admitted to hospital had used cannabis at least once in their lifetime and nearly one-third were positive for THC on urine drug screening. This suggests that cannabis use is prevalent in New Zealand's acute admission patients. It is interesting to note, however, that the use of other drugs was rare. These preliminary data suggest that those with a positive THC urine test were less likely to be thought-disordered, deluded or suspicious. Psychiatrists should be aware that cannabis use is widespread and consider using urine testing as part of assessment. Further studies are needed to examine in more detail the role of cannabis in the presentation of psychiatric illness.