This study in Finland tracked 6,325 people from birth to age 33 (a single cohort born in 1986). It was published in the journal Addiction in March 2022.
By age 33, 1% of this cohort was diagnosed with bipolar disorder and 5.6% reported early cannabis use. Bipolar disorder was measured via bipolar disorder-related diagnostic codes (ICD-10: F30.xx, F31.xx) collected from the Care Register for Health Care 2001-18. Cannabis use was self-reported.
The initial analysis did show a statistically significant correlation. However, after the authors adjusted for frequent alcohol intoxication, daily tobacco smoking and lifetime illicit drug use, the associations were determined to be NOT statistically significant.
So, the authors concluded that cannabis alone was not responsible for diagnosed bipolar disorder, given the tendency of this group to use other intoxicants like alcohol and other drugs.
"In Finland, the positive association between early cannabis exposure and subsequent development of bipolar disorder appears to be confounded by other substance use."
The full-text paper is available here from the Wiley Online Library.