Answer
It blocks vasopressin in the kidneys, causing more water to leave the body as urine. Thirst is an expected effect, but severe thirst with dizziness, fainting or confusion needs prompt assessment.
Sources for this answer
- Tolvaptan 15mg and 45mg tablets — Current selected SmPCemc · Summary of Product Characteristics
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.