Poisoning due to excessive levels of theophylline-containing compounds in the blood, characterized by nausea, vomiting, agitation, cardiac arrhythmias, and, in some instances, seizures or death. For young patients with asthma, theophylline levels exceeding 20 mg/dl are typically toxic; even lower levels (e.g., 15 mg/dl) may produce toxic effects in people over 60. Theophylline levels above 30 mg/dl have a high likelihood of adverse effects at any age.