Which of the following does not increase tricyclic antidepressant concentrations?

Cigarette smoking does not increase tricyclic antidepressant (TCA) concentrations due to its impact on the liver's metabolic enzymes. Specifically, substances found in cigarettes can induce cytochrome P450 enzymes, notably CYP1A2. When these enzymes are induced, the metabolism of various drugs, including tricyclic antidepressants, can be increased. As a result, smokers may require higher doses of TCAs to achieve therapeutic effects, as the presence of this enzyme can lead to lower overall drug levels in circulation.

In contrast, the other choices, such as clozapine, haloperidol, and methylphenidate, have known pharmacokinetic interactions with TCAs that can lead to increased concentrations. Clozapine can inhibit certain enzymes that metabolize tricyclics, potentially leading to toxicity. Haloperidol can also affect the metabolism and elimination of TCAs, while methylphenidate may reduce the clearance of these antidepressants, resulting in elevated blood concentrations. Thus, cigarette smoking's role in inducing metabolism distinguishes it from the other options listed.