A patient with drug dependence mentions experiencing sounds as colors. What is his likely drug of choice?

The experience of perceiving sounds as colors is known as synesthesia, which is often associated with the use of hallucinogenic substances. Lysergic acid diethylamide (LSD) is known for inducing such perceptual alterations, which can include synesthetic experiences. LSD acts primarily as a serotonin receptor agonist, which can significantly alter sensory perception and cognitive processes, leading to the vivid and unusual experiences characteristic of hallucinogens.

While other substances might create various effects, they are less commonly associated with this specific symptom. For example, cocaine is a stimulant that primarily affects energy and mood, while heroin is an opioid that generally leads to sedation and pain relief, without such perceptual changes. MDMA, while it does alter sensory experiences and emotional perception, is typically not linked to synesthesia to the same degree as LSD. Thus, the profile of sensory alterations resulting from LSD makes it the most likely drug of choice for a patient experiencing sounds as colors.