By all accounts, a slow and painful way to go.
While the entire plant is toxic, the most dangerous part is the seeds, which contain the highest alkaloid concentration. It contains eight piperidine alkaloids, with the most potent one being coniine. Coniine is a neurotoxin that disrupts the function of the peripheral nervous system, causing respiratory paralysis. Initial symptoms include nausea, vomiting, abdominal pain, hypertension, tachycardia, tremor, and temperature increase. Symptoms progress toward central nervous system depression, coma, muscular weakness and/or paralysis, with respiratory distress. The concentration of alkaloids found in each plant varies depending on age of plant, season, and precipitation. No antidote specifically exists for hemlock.