MedicineNet confirms that naproxen, the active drug ingredient in Aleve, thins the blood. It should not be used in conjunction with any blood-thinning medications, such as warfarin.
MedicineNet advises people who have a bleeding condition, such as a bleeding ulcer, not to use naproxen, as the bleeding may be exacerbated by the blood-thinning effect. People at risk for stroke must also avoid naproxen due to the blood-thinning effect. Naproxen has a tendency to reduce the efficacy of blood pressure medications as well, resulting in a possible spike in blood pressure. Naproxen is classified as an NSAID and is contraindicated for those with NSAID allergies.