People with bipolar disorder are more likely to use drugs than people with any other mental illness yet the effects of drugs on bipolar disorder are not clearly understood. A team of researchers led by Michael J. Ostacher from Massachusetts General Hospital studied 3,750 people with bipolar disorder over two years. Contrary to the researchers' expectations the people who used drugs were no slower to recover from the depressive phase of the illness than those who didn't. However, the people who used drugs were more likely to switch straight from a depressed to a manic state with no period of 'normality' in between.
Ostacher, Michael J. ... [et al] - Impact of substance use disorders on recovery from episodes of depression in bipolar disorder patients: prospective data from the systematic treatment enhancement program for bipolar disorder (STEP-BD) American Journal of Psychiatry March 2010, 167(3), 289-297