A drug used in asthma treatment and prevention. Theophylline is a bronchodilatory drug and comes in various forms such as short-acting tablets and syrups, as well as longer-acting sustained release capsules and tablets. It can also be given intravenously if a serious asthma attack is taking place. The dose of Theophylline must be closely monitored as too little has little effect, while too much can cause side effects such as abnormal heart rhythms, nausea, insomnia and seizures.