Smoking is the major preventable cause of chronic medical diseases and death worldwide. Significant efforts led by World Health Organization (WHO) and several countries have been made to help current smokers to quit, particularly in the developing countries, where their smoking rates continued to rise. Clinical practice guidelines were therefore developed to guide all healthcare personnel in those countries to provide treatment for smokers. However, despite that, only a small number of smokers were able to quit successfully. A large number of smokers who are poor and heavily addicted to nicotine remain and have no access to any effective smoking cessation aids. Although there are several effective medications available for smoking cessation nowadays, the high price of them completely keeps poor smokers away from using them. Alternative and cheaper smoking cessation aids are therefore necessary. During the past decade, certain herbal remedies have been introduced in smoking cessation in Thailand. One of them that have been used widely without any supporting evidence is the fresh lime (Citrus aurantifolia), the well-known native citrus to Southeast Asia. It is anecdotally claimed to be effective and safe in smoking cessation. The present study was designed to determine the efficacy and safety of fresh lime as a smoking cessation aid compared with those of nicotine gum.