Theophylline is a drug in the methylxanthine family, specifically 1,3-dimethylxanthine. Its empirical formula is C7H8N4O2 and it is an isomer of both theobromine and paraxanthine. It is also structurally related to caffeine, with its structure being that of a caffeine molecule missing a single methyl group. Its structure is a derivative of adenine, one of the most important molecules in a variety of biological processes. Theophylline was first isolated from tea leaves, and this is how it received its name. While theophylline can be found in trace amounts in coffee, tea and chocolate, pharmaceutically available theophylline is synthesized from caffeine.
Although I have not found any exhaustive treatment of the subject, scientific or otherwise, the best way to describe theophylline's effects on the human body is as a less fun, more dangerous caffeine. Theophylline is a stimulant like caffeine, but appears to have more of a "physical" effect and less of a "mental" effect. Its side-effects and risk of overdose also seem to be greater. Theophylline is a bronchodilator and is used in the treatment of asthma, emphysema, chronic obstructive pulmonary disease and other pulmonary conditions. It was at one time a first-line strategy for the treatment of asthma, but has since become less popular. This is not due to a lack of effectiveness, but because of the severity of its side effects: insomnia, irritability, nausea, tachycardia and sometimes seizures. Currently, theophylline is used mostly for difficult pulmonary conditions, such as sleep apnea in premature infants, and may work as a pain killer in some specific syndromes. There is a good chance that theophylline is not researched as heavily as it could be as a therapeutic agent, due to the fact that it is long out of its patent period and has few commercial prospects. Also, of course, its side-effects and high risk of overdose make it a difficult drug.
What is interesting to me is that, as we see so often in organic chemistry, a very slight change makes a gigantic difference.. The difference between caffeine and theophylline is merely the removal of a single methyl group. And yet, that change is the difference between the world's most popular psychoactive drug and a medication only used to treat certain niche conditions.