Aminophylline Injections may cause nausea, vomiting, hyperglycemia, restlessness, muscle twitching, insomnia, diarrhea, polyphagia, polydipsia, and polyuria, but these side effects are typically the cause of a mild overdose, as this drug can be toxic in higher doses. A higher level of toxicity may cause seizures and cardiac arrhythmia, and if high enough, even death. Due to the narrow therapeutic window in which this drug operates, it is crucial to be very careful when administering the drug. Also, if giving the drug intramuscularly, know that this will be very, very painful for your pet.