Methylxanthines, namely caffeine, theobromine and theophylline are found in several beverages and food products such as coffee, cocoa, tea and cola drinks. These substances can provide benefits to humans but also prove harmful mainly when consumed in high amounts. However, the beneficial and deleterious effects of methylxanthines are controversial in research findings, largely because of inconsistencies within research conducted and factors associated within studies. For instance, some authors have reported methylxanthines could have potential antioxidant activity under physiological conditions but opposing studies do not support these claims. Factors that influence the effects of methylxanthines include age, gender and health status of the individual. In addition to these, other factors such as doses, consumption of drugs or pharmaceuticals, alcohol and tobacco habits and diet also play a role in methylxanthine effects and metabolism. Therefore, our review is to provide a clearer understanding of the beneficial/adverse effects of methylxanthines in humans. A description of the physiological and toxicological effects of these methylxanthines is provided.