Yup...digoxin and spironolactone are both common medications used in the
treatment of congestive heart failure; however, many people are unaware that
these two medications interact with each other. When used together
spironolactone increases digoxin levels. Although the exact mechanism is
unknown it is likely due to the inhibition of active tubular secretion of digoxin
(leading to decreased digoxin clearance). 
 
When these meds are given together, ask patients to watch for potential signs
of digoxin toxicity. Early signs include anorexia, nausea, vomiting. Other
signs of toxicity include headache, fatigue, drowsiness, generalized muscle
weakness, and ocular effects (yellow/green halo). 
 
Rule of Thumb  When a drug is added to digoxin therapy that affects its
clearance, digoxin levels can be rechecked in 2 days to monitor for any
changes. Digoxin Levels should be taken at a minimum of eight hours
after the digoxin dose and desired levels are 0.65-1.04 nmol/L.