Chocolate is the best-known household hazard for dogs, and the risk is routinely described as if all chocolate were equivalent. The relevant compound is distributed very unevenly.
The compound responsible
The primary agent is theobromine, a stimulant belonging to the same chemical family as caffeine, which is also present in smaller amounts.
Both act on the nervous system and the heart, increasing rate and excitability, and both interfere with normal muscle relaxation.
The compounds are not uniquely harmful to dogs; the difference is how slowly dogs break them down compared with people.
Why metabolism is the whole difference
Humans clear theobromine relatively quickly, so ordinary quantities produce no lasting effect.
Dogs process it far more slowly, which means it remains in circulation for many hours and accumulates during that time.
The long clearance is also why signs can develop hours after eating and why they persist well beyond the point at which the chocolate itself has gone.
Where the compound is concentrated
Theobromine sits in the cocoa solids. Products are therefore ranked by how much of that they contain rather than by how much they weigh.
Cocoa powder and dark chocolate carry the most, milk chocolate substantially less, and white chocolate contains almost none because it is made from cocoa butter without the solids.
Cocoa mulch used in gardens is a frequently overlooked source, as it is made from the residue of processing and can be highly concentrated.
Why body size changes the picture
Risk depends on the amount consumed relative to the dog's size, so an identical quantity is a much larger exposure for a small dog.
A piece of dark chocolate that would be inconsequential to a large dog can be significant for a small one.
This is why any exposure is assessed on the product type, the amount and the dog's weight together rather than on any single one of them.
What to do and what not to
The correct response is to contact a veterinarian or an animal poison service promptly with the packaging to hand, rather than waiting for signs to appear.
Timing matters, because there is a window in which intervention is far more straightforward than it becomes later.
Do not attempt to induce vomiting at home. Methods circulated online carry their own risks, and whether it is appropriate at all depends on what was eaten and when, which is a professional judgement.