Ritalin also has sudden death syndrome as a possible complication. All stimulant medications are.... Well, stimulatory. They increase your physical "readiness" by increasing heart rate, blood pressure, etc. This is normally not a dangerous effect (although it might be uncomfortable), however it can be dangerous in a person with an actual heart defect. To repeat my previous example of a 5ht2b-induced cardiac cell proliferation: mitral valve prolapse is a heart defect where the leaflet separating your left ventricle and left atrium doesn't properly close during ventricular contraction--resulting in partial ejection of blood backwards into the atrium during each heart beat. This means that your heart has to work harder, because it needs to apply enough pressure to propel blood through your body while also propelling some blood backwards. additionally, this increased heart load causes compensatory increases in the contractile strength of your left ventricle--corresponding to a greater pressure of backflow. increasing pressures in the left atrium can eventually preventing freshly oxygenated blood from entering via the pulmonary veins from lungs. This in turn causes hypertrophy (increased size) of the left atrium and ventricle in order to improve pulmonary flow. This is a condition which can take years to become significant enough to warrant clinical treatment--or may never require treatment at all. However, the eventual outcome of these changes is that the hypertrophic ventricle is no longer able to fully clear itself in the time period between heartbeats (ie. Decreasing ejection fraction), increasing risks of congestive heart failure, etc. The key here is that decreasing ejection fraction actually decreases the volume of blood being moved. This means that stimulants could exacerbate a condition like this by increasing heart rate, thereby decreasing the ejection fraction of each ventricular contraction and leading to congestive heart failure.
That's how stimulants can "cause" sudden death. But they're not really the cause. In healthy individuals, stimulants are fairly well tolerated. The only significant difference in safety profile between amphetamine and ritalin has to do with the fact that amphetamine is a monoamine releaser, while ritalin (methylphenidate) is a "mere" reuptake inhibitor. At high levels of reuptake inhibition, your body can compensate by sharply reducing the release of new monoamines into the synapse in response to stimuli. However amphetamine, by directly reversing monoamine transport, doesn't require any "natural" release of monoamines to exert its effects. This basically means that solo-amphetamine-intoxication can be lethal at lower doses than solo-methylphenidate intoxication.
