Chocolate toxicosis results from ingestion of excessive amounts of methylxanthine-containing chocolate products. Clinical effects include vomiting, restlessness, agitation, hyperthermia, tachycardia, and, in severe cases, seizures.
The amount ingested that can cause toxicity depends on the size of the dog and the type of chocolate, with darker and more bitter chocolates being the most toxic. If your dog accidentally ingests chocolate, contact an emergency veterinarian immediately.
Chocolate ingestion is one of the most common and dangerous accidents for dogs. Understanding the risks, symptoms, and emergency procedures allows owners to act effectively and greatly increases the chances of recovery.

Further down the line, dogs can develop a rapid heart rate, heart arrhythmias, muscle tremors, seizures, and heart failure. Signs can take several hours to develop and can last for a few days. Long-Term Effects of Chocolate Ingestion in Dogs.
Chocolate toxicity results from the methylxanthines causing gastrointestinal (e.g. vomiting), cardiovascular (e.g. tachycardia) and central nervous signs (e.g. agitation and seizure). A UK study, Noble et al.

No, dogs should never be allowed to eat chocolate. Chocolate contains certain ingredients that a dogs digestive system cannot break down, causing them to develop gastrointestinal, neurological, or cardiovascular issues.
Yes, chocolate is toxic to dogs. Chocolate contains theobromine and caffeine, which are similar compounds and cause similar clinical signs such as an increase in heart rate, blood pressure, agitation, tremors, and seizures. While rarely fatal, chocolate ingestion can result in...
Chocolate contains theobromine and caffeine, both methylxanthines that dogs metabolize slowly. Even small amounts can cause vomiting, diarrhea, rapid heart rate, seizures, and in severe cases, death. The darker and more concentrated the chocolate, the higher the risk.