To put it bluntly, Harford is clueless about the subject which he writes.

Heroin is an extremely addictive drug. Once a person becomes addicted to heroin, the continued use usually is more to offset the brutal effects of withdrawal than to get high.

Withdrawal from heroin and other opiates can set in within hours of cessation of use and often involves violent vomiting and diarrhea, tremors, hot and cold sweats, skin crawling sensation, insomnia, and, in some cases, hallucinations. Withdrawal, in some cases (e.g. people already in poor health), can even lead to death. These symptoms typically last for five to ten days. In fact, heroin addiction can be so extreme that it requires another highly addictive drug, methadone, to safely quit. (Methadone withdrawal occurs slower than heroin withdrawal, though, so a patient can slowly reduce dosage to eventually quit).

The notion that a "rational" heroin addict might consider not only current costs, but future costs of the drug is patently absurd. This simply overlooks the chemical side of addiction--that an addict is thinking about the next fix and how to get it, not the freakin' price of it. An addict will lie, cheat, steal, prostitute, and/or more to get the fix.

Of course, caffeine is also highly addictive and withdrawal from it can have effects similar to opiate withdrawal. However, the similarity is in the nature (headaches, vomiting, muscle pains, etc.) not the extremes.

The rationale to quit using heroin, I would argue, has little or nothing to do with economics, rather with the illness of addiction itself. Often an addict will realize that s/he is no longer in control of her or his own life and decide to try to quit. Other times, concerned friends or family step in to help them quit.

Finally, why would only addicts reduce use in the face of a liquor tax? Could it be that they could afford less? And since overall consumption did not decrease at the same rate as liver cirrhosis, it would appear rationalism does not dictate use. Perhaps reviewing a similar study with tobacco taxes is in order.