Patients should not take nitrates within 24 hours after they have taken a medication to increase their sexual potency (e.g., Viagra). People should also avoid nitrates if they have been diagnosed with low blood pressure (hypotension). Before using nitrates, patients should inform their physician if they have had allergic or other types of reactions to nitrates or nitrites.

Other medical conditions may affect the use of nitrates. According to the National Institutes of Health, a physician should be informed of any other disorders, especially:

• Recent heart attack. Nitrates can worsen problems associated with a heart attack by lowering blood pressure.
  
  
• Hypertrophic cardiomyopathy. In this condition, nitrates may decrease the heart's effectiveness.
  
  
• Pericarditis.
  
  
• Recent stroke or head injury. Nitrates can worsen these conditions by increasing pressure in the brain.
  
  
• Severe anemia.
  
  
• Glaucoma.
  
  
• Kidney or liver disease.
  
  
• Overactive thyroid gland (hyperthyroidism).