Organic substances containing nitrogen are widespread throughout the world. Amongst these are the methylxanthine derivatives caffeine, theobromine and theophylline. These are very closely related structures with similar pharmacological properties. They can occur in as many as 60 plant species. They are often referred to as alkaloids. Theobromine is the major alkaloid in cocoa and is said to contribute towards the typically bitter taste of cocoa.

A survey by MAFF of methylxanthines in a range of drink and chocolate products showed that theobromine levels were low, with the exception of chocolate products.

Theobromine concentrations in the powdered drinks were in the range 85 - 590 mg/l with a mean value of 367 mg/l. Theobromine concentrations in the chocolate milk drinks ranged from 141 to 371 mg/l. The chocolate mousses had theobromine contents of 386 and 651 mg/kg. In the chocolate bars theobromine levels ranged from 1300 to 4740 mg/kg. Higher cocoa content (70% cocoa) chocolate bars had a theobromine concentration of 10370 mg/kg.

The variable levels of theobromine found in chocolate bars are thought to reflect the variable concentrations found in cocoa beans. The amount of methylxanthines in cocoa beans depends on various influencing factors: processing procedures, genotype, geographical origin, degree of maturity at harvest and cocoa bean weight.

Methylxanthines are rapidly absorbed by humans and theobromine levels are halved between 6 and 10 hours after consumption. Methylxanthines are mild stimulants but theobromine has almost no effect on the central nervous system. All methylxanthines have been shown to relax the smooth muscle of the bronchi in the lungs, but it is unlikely that this effect is noticeable when they are consumed as part of food or drink. All methylxanthines have a diuretic effect, but this is only noticeable when they are consumed with large quantities of water.

Research has found that pets, especially dogs, are sensitive to theobromine. Several cases of dog poisoning have been reported and the amount of chocolate eaten varied between 100g and 1kg. The dogs' symptoms were mainly neurological (trembling and convulsions), but others such as diarrhoea also occurred. The concluding evaluation of methylxanthines by dieticians and toxicologists is that plants containing methylxanthines have been consumed for as long as can be remembered without having led to damage to health. Scientific research has shown that the consumption of these substances has no consequence as long as consumption is not excessive. Methylxanthines in cocoa products have no negative effects on the health of humans because their amounts in chocolate and other processed foodstuffs containing cocoa are so low that they account for only a very small proportion of the human diet.