Prolonged hot weather can influence your INR

The package insert of warfarin lists several environmental factors that affect INR's. Weather conditions, especially intense heat, have been known to alter patients INR.1

Fluctuations in temperature and humidity have a profound effect of patients, particularly the elderly. Heat related stress and uncomfortable humidity (dew point) typical in the summer season elevates patients' INR. While an occasional hot or humid day will not have a profound effect on a given INR, prolonged exposure to high heat and humidity will stress warfarin patients and INR's can easily drift above target INR levels.

Hot weather over a prolonged period of time has been shown to elevate INR values - even on the most stable patients. The additional effect of humidity and high dew points make it virtually impossible to cool the body and results in the potential for patients to be higher than normal.

Make every effort not to become over-heated, drink plenty of non-alcohol beverages and keep close to your physician's orders while keeping all summer INR tests. Contact your doctor for further information.

  1. Bristol-Myers Squibb Company. Medication Guide for Coumadin Tablets and Coumadin for Injection [Package Insert]. Princeton, NJ: Bristol-Myers Squibb Company. 2009. 15-16.