Which of the following is a stimulant?

Methamphetamine is classified as a stimulant because it enhances the activity of certain neurotransmitters in the brain, particularly dopamine and norepinephrine. Stimulants are substances that increase brain activity, leading to heightened alertness, increased energy levels, and improved cognitive function. Methamphetamine is well-known for its potent effects, often leading to euphoric feelings and increased physical activity while also presenting significant risks for addiction and adverse health effects.

The other substances listed serve different purposes: Lorazepam is a benzodiazepine and acts as a sedative or anxiolytic, calming the nervous system. Fluoxetine, an SSRI, is primarily used as an antidepressant and works by increasing levels of serotonin in the brain, not stimulating its activity. Imipramine, a tricyclic antidepressant, also serves primarily to treat depression and works differently than stimulants. This distinction in how these substances affect the central nervous system helps affirm methamphetamine's classification as a stimulant.