Several large, long-running studies have found that smoking significantly increases your risk of Alzheimer's, as does high exposure to secondhand smoke. Smoking, both in old age and in adolescence, is also associated with lower IQs and lower cognitive performance. It appears, however, that this is largely recoverable if you give up smoking.
Smoking has a particularly negative effect in conjunction with alcohol (and unfortunately they are often found in tandem). While moderate drinking can in some circumstances have positive effects on the brain, this is probably not the case for those who smoke. Moreover, smoking makes it much harder for the brain to recover from the effects of alcohol abuse and the damage done to the brain by heavy alcohol consumption is likely to be much worse if the individual is a smoker.