By Alastair Culham

A ball and stick chemical model of one six angled ring attached to one five angled ring, each with two nitrogen moleculaes. The six membered ring has two oxygen molecules double bonded to carbon atoms.

In 2014 I introduced the food of the gods, Theobroma cacao, as the source of chocolate, that staple of Christmas excess, in the 16th Advent Botany post.  Today, in the second for 2019, I explore Theobromine, perhaps the best known chemical compound in chocolate, and one of the many chemicals that make it so popular.

Theobromine, is a xanthene alkaloid like caffeine, and is formed during the metabolic breakdown of caffeine as well as ocurring naturally in cocoa and some other plants.  It reduces sleepiness by blocking adenosine receptors, it dilates blood vessels and relaxes smooth muscle tissue in the lungs.  The effects are long-lasting with a half life of around three hours in humans and it has been shown to have a mood improving effect.  This contrasts with 5-6 hours half life for caffeine.

Ball and stick models of the caffeine and theobromine molecules.
Ball and stick models of Caffeine (left) and Theobromine (right). Carbon = black; Nitrogen = blue; Oxygen = red; hydrogen = white.

Theobromine can be toxic in high concentrations (Finlay & Guiton, 2005) but the LD50 (median lethal dose) is around 0.5-1 gram per kilo of body weight so a 70 kg (11 stone) adult would need to eat 35-70g of Theobromine (ca 10kg of dark chocolate) to be fatal.

A bar of dark chocolate seen from the top. It is surrounded by gold wrapping foil.
Dark chocolate (Photo: jackmac34 (

Before that, the vomiting and diarrhoea of Theobromine toxicity would have cut in.  This is conjecture, of course, as the estimates are based on experiments with rats.  Human experiments have not been conducted.  I could find no reported case of death by chocolate in humans.  In cats and dogs the lethal dose is lower (ca 0.3g Theobromine per kg body weight) and they metabolise the theobromine more slowly (a half life of 18 hours has been reported) so 2 kg of chocolate could deliver the fatal dose to a small dog.  Again, the symptoms of the toxicity are likely to clear the chocolate from the system before that point is reached.

So, please enjoy your chocolate sweets over the Christmas period but don’t over-eat!


Anon Does Chocolate contain drugs? — Molecules found in Chocolate, The Science of Cooking (Accessed 31st November 2019)

Finlay, F., & Guiton, S. (2005). Chocolate poisoning. BMJ : British Medical Journal, 331(7517), 633.


Share this: