Finding an appropriate Coumadin® dosage can be very difficult. The level of the drug in the body and the therapeutic effect that it has are affected by a number of different factors. Patients on Coumadin® typically require regular monitoring to ensure they are receiving the correct dose, as high levels of the drug in the blood can cause dangerous outcomes such as bleeding in the brain. Factors such as a patient's genetic makeup, dietary intake, other medications taken, and underlying medical conditions can all affect the dosage of this drug.
One of the most important factors affecting Coumadin® dosage is a person's genetic makeup. Some people may have abnormal variations of proteins that result in an innately lower ability to break down this medication. They require lower doses of Coumadin® because the drug tends to stay in their systems for a longer time as compared with people who have normal proteins. Although doctors can check for these genetic variations by sending the patient's blood to specialty labs, in practice they simply monitor the patient's response and decrease the dose if indicated.
Another critical aspect affecting Coumadin® dosage is the patient's diet. This medication works by inhibiting vitamin K, and as a result foods that are high in vitamin K can overrule the effect of the drug and alter its effectiveness. Foods high in this vitamin include spinach, collard greens, broccoli, grapes, parsley, and avocado. Patients are advised either to avoid these foods, or to eat a consistent amount of these foods on a weekly basis.
Taking other medications can also affect the required Coumadin® dosage. Some medications cause an increase in the breakdown of Coumadin®, thus decreasing its effectiveness. These medications can include rifampin, carbamazepine, or barbiturates. Other medications can increase the therapeutic effect of Coumadin®, which can be dangerous because this could lead to bleeding. Examples of these medications include cimetidine, fluoxetine, metronidazole, and amiodarone.
Patients with certain underlying medical conditions also might require an adjusted Coumadin® dosage. For example, patients with poor liver function might require a lower dose in order to obtain a therapeutic effect. Patients who have gastrointestinal problems and cannot properly absorb material in the digestive tract might require higher doses. The presence of congestive heart failure often means that patients will require lower amounts of the medication.