While anecdotal evidence and some case studies suggest cannabis may benefit those with bipolar disorder, studies have reported mixed results.
The endocannabinoid system is thought to play a role in bipolar disorder, chiefly due to its role in regulating emotions. As cannabis affects this system, it may make sense that cannabis can either improve or aggravate symptoms of bipolar disorder. Given the small amount of research conducted on the link between cannabinoids and bipolar disorder, however, more is needed before any conclusions can be drawn.
That said, most studies to date seem to show that cannabis aggravates bipolar disorder. A 2-year observational study of 1922 patients, for example, found that patients who used cannabis during manic episodes had poorer functioning and a higher risk of recurrence than those who either did not use the substance or who stopped using it during manic episodes.
Meanwhile, a six-day diary-based study conducted on 24 participants found that while cannabis improved positive mood among those with bipolar disorder, it also increased symptoms of mania and depression.
On the flip side, however, a study of 133 participants with bipolar disorder found that those who used cannabis had better neurocognitive function than their non-cannabis-using counterparts. This included better attention, logical thinking ability, and memory.
In tandem with the positive outcomes noted by this study, doctors have reported positive outcomes from patients using cannabis to treat the condition. In these instances, patients reported the substance helping them with their manic rages, depressive episodes, or both, and that outcomes from cannabis were better than those from conventional treatments for the condition, such as lithium.
To conclude, while further research is needed to properly confirm the effects of cannabis on bipolar disorder, given the connection between the endocannabinoid system and emotional regulation, it is likely to have some effect. What can influence the quality of this effect, however, is a subject a further study.