Which of the following drugs can cause both stimulation and depression?

PCP, or phencyclidine, is a dissociative anesthetic that can produce effects that range from stimulation to depression, making it unique among the substances listed. At lower doses, PCP often leads to feelings of euphoria, increased energy, and heightened sensory perception, which are characteristic of stimulant effects. However, as the dosage increases, individuals may experience significant dissociation, hallucinations, and ultimately depressive effects, including sedation and decreased responsiveness. This duality in its effects is documented and recognized in clinical settings, making PCP a drug capable of causing both stimulation and depression.

In contrast, the other substances have predominant effects that lean more toward either stimulation or sedation. Marijuana typically induces relaxation and mild euphoria but does not have the pronounced stimulating effects seen with PCP. Alcohol is primarily a depressant and tends to lead to sedation, though it may cause initial feelings of euphoria and lowered inhibitions. Caffeine, mainly known as a stimulant, generally does not produce depressant effects. Hence, PCP's capacity to induce both stimulation and depression distinctly qualifies it as the correct answer.