5-hydroxytryptamine (5-HT or serotonin) is a charged molecule and must be transported across biological membranes. Enzymes that catabolize 5-HT are all intracellular; therefore, 5-HT inactivation requires a high affinity transporter, known as serotonin transporter (SERT or 5-HTT). In the central and enteric nervous systems, SERT is located in serotonergic neurons; however, these neurons are not present in the gastrointestinal mucosa, where 5-HT initiates peristaltic and secretory reflexes. Instead, SERT is expressed by enterocytes. The severity of gastrointestinal effects caused by drugs that inhibit SERT, such as tricyclic antidepressants, selective serotonin reuptake inhibitors and cocaine, does not usually prevent their therapeutic or recreational use because backup transporters and alterations in receptor gene expression allow the gut to adapt, albeit imperfectly, to their toxicity.