Which of the following substances is noted for being used as over-the-counter nasal inhalers?

The substance that is noted for being used in over-the-counter nasal inhalers is methamphetamine/amphetamine. These stimulant compounds can act as decongestants in certain formulations, primarily by causing vasoconstriction of the blood vessels in the nasal passages, leading to reduced swelling and improved airflow. Historically, substances like pseudoephedrine, which is a structural analog of methamphetamine, have been commonly used for this purpose, but tightening regulations have made it necessary to control the precursor chemicals.

In the context of the other options, serotonin is a neurotransmitter that primarily functions within the brain and is not associated with nasal inhalers. Tricyclic antidepressants are a class of medications used to treat depression and are not utilized in the treatment of nasal congestion. Venlafaxine is an antidepressant that falls under the category of serotonin-norepinephrine reuptake inhibitors (SNRIs) and is similarly unrelated to nasal inhalers. Therefore, the distinction of methamphetamine/amphetamine as it relates to its historical use in nasal inhalers clarifies its relevance in this context.