Patients with ADHD are twice as likely to have problems with substance abuse as those without ADHD. The reasons for this are numerous and vary for each person, and usually include genetic risk factors. It is also often the case that patients with ADHD are more impulsive in most regards, including with the ability to modulate substance usage. However, even when patients feel that their substance usage is related to their ADHD, substance use disorders are best addressed and treated as problems in their own right; treatment of ADHD does not by itself seem to reduce rates of substance abuse.
Some forms of substance abuse, particularly regular marijuana usage, can present with symptoms similar to ADHD. Chronic cannabis usage is associated with neurocognitive effects that include deficits in attention and working memory, slowed mental processing and difficulties with decision making and impulsivity. These deficits have been documented to last several weeks past the last time of marijuana usage. Some patients report that they use marijuana or other substances to self-medicate their attentional difficulties. Given what is known about the effects of cannabis on cognition, it is difficult to postulate a physiologic mechanism whereby marijuana could treat a primary attentional deficit.
For these reasons, the ongoing usage of substances, particularly marijuana or stimulants (such as methamphetamine or cocaine), makes it quite difficult to have diagnostic clarity prior to beginning treatment. Those substances also make it challenging to assess the true benefits of a medication once ADHD treatment has been initiated, since ADHD medications are known to be of limited benefit while there is ongoing substance usage. In all cases it is recommended that patients be off all substances of potential abuse before starting medication treatment for ADHD.
If you think you may have ADHD but are still using excessive alcohol, marijuana or other drugs, I would be glad to provide you with some referrals to help you try to address your substance use before trying to treat your attentional difficulties.
There is quite a large medical literature on this topic. I list some examples here for those interested in reading deeper into the topic:
Neuropsychological deficits associated with cannabis use in young adults. Grant JE, Chamberlain SR, et al. Drug and Alcohol Dependence. 2011 Sep 13.
Adolescent substance abuse: the effects of alcohol and marijuana on neuropsychological performance. Thoma RJ, Monnig MA, et al. Alcohol Clinical and Experimental Research. 2011 Jan;35(1)
Chronic cannabis users show altered neurophysiological functioning on Stroop task conflict resolution. Battisti RA, Roodenrys S, et al. Psychopharmacology. 2010 Dec;212(4):613-24.
Executive function deficits in short-term abstinent cannabis users. McHale S, Hunt N. Human Psychopharmacology. 2008 Jul;23(5)
The influence of marijuana use on neurocognitive functioning in adolescents. Schweinsburg AD, Brown SA, et al. Drug Abuse Reviews. 2008 Jan;1(1)
The relationship between non-acute adolescent cannabis use and cognition. Harvey MA, Sellman JD, et al. Drug and Alcohol Reviews. 2007 May;26(3)
Neuropsychological deficits in long-term frequent cannabis users. Messinis L, Kyprianidou A, et al. Neurology. 2006 Mar 14;66(5)
Attributes of long-term heavy cannabis users: a case-control study. Gruber AJ, Pope HG, et al. Psychological Medicine. 2003 Nov;33(8)
Cognitive measures in long-term cannabis users. Pope HG Jr, Gruber AJ, et al. Journal of Clinical Pharmacology. 2002 Nov;42(11 Suppl)
Dose-related neurocognitive effects of marijuana use. Bolla KI, Brown K, et al.Neurology. 2002 Nov 12;59(9)