With cigarettes and alcohol, laying aside what levels may trigger evasion, we can tax it if we want to exploit it or to reduce its use. The level of taxes that makes sense as a minimum (and probably left there) is that which recovers the other costs (including externalities) associated with the use of the substance in question. Assuming a higher level is possible without triggering evasion, and assuming some might have an interest in knowing the answer to the question, the effect of price increases (through the taxes) on use reduction (quantifying the effect) can be obtained empirically.

That leads me to remark on a tangential matter. If Obama wanted to be serious, he'd tax tobacco and alcohol much more than is now the case, if he was interested in reducing harm to people's health (which is not limited solely to individual consumers). However, for Obama and his administration to be consistent, he would have to engage in “cap and trade,” the same game to be sought not for real (serious) air pollution but to control “greenhouse gas” emissions and exert control over people and industry thereby. For Obama to be consistent, he would not impose or raise taxes on cigarettes, but would play “cap and trade” and give various tobacco companies special quotas of production they could meet, Soviet-style.