Which of the following is a common side effect of caffeine toxicity?

Caffeine toxicity can result in a variety of harmful physiological effects, and seizures represent one of the more significant concerns. Caffeine is a central nervous system stimulant, and excessive intake can lead to increased neuronal excitability, which can ultimately precipitate seizures. This reaction may be especially pertinent in neonates, who are more susceptible to the effects of stimulants due to their developing nervous systems and lower tolerance for such agents.

In contrast, decreased heart rate is generally associated with the calming effects of many medications like sedatives or in certain conditions but is not a feature of caffeine toxicity, where elevated heart rates are more common. Weight loss can occur due to various reasons in neonates but is not directly linked to caffeine toxicity. Caffeine can actually affect the gastrointestinal system, potentially resulting in increased motility and diarrhea rather than constipation. Thus, the occurrence of seizures is a critical and direct consequence of caffeine toxicity, making it the correct choice in this scenario.