Tolerance to organic nitrates: evidence, mechanisms, clinical relevance, and strategies for prevention. OBJECTIVE: To review the available information about nitrate tolerance, its potential mechanisms, clinical implications, and strategies for prevention. DATA IDENTIFICATION: A survey of the National Library of Medicine MEDLINE database and bibliographies of the reviewed articles. STUDY SELECTION AND DATA EXTRACTION: Studies were selected from the English language literature with an emphasis on recent studies and, when available, randomized placebo-controlled studies. Old studies were selected on the basis of their historical value and originality. A total of 134 retrieved articles were considered relevant and were reviewed in depth. RESULTS: The available information about the experimental as well as the clinical evidence for tolerance to organic nitrates has been summarized. In addition, information related to potential mechanisms, clinical implications, and possible methods for prevention have been reviewed. CONCLUSIONS: Evidence indicates that prolonged in-vitro exposure to organic nitrates, continuous intravenous or topical administration of nitrates, and frequent in-vivo oral dosing result in the rapid development of tolerance to the peripheral as well as to the coronary vasodilatory effects of the drugs. This phenomenon leads to the rapid attenuation of the hemodynamic and anti-ischemic effects of nitrates in patients with ischemic heart disease or congestive heart failure, or both. Tolerance development seems to be dose- and time-dependent, and its main mechanism seems to be a depletion of sulfhydryl groups at the vascular cell. Although the repletion of sulfhydryl groups with the use of sulfhydryl-containing drugs may help to prevent tolerance, the efficacy and safety of this approach requires further evaluation. Intermittent therapy allowing a sufficiently long, daily nitrate-washout interval seems to be the most effective and the most safe strategy currently available for the prevention of nitrate tolerance. 