The maximum dosage for over the counter aspirin is 4,000 milligrams total every 24 hours. Aspirin becomes severely toxic if too much is consumed at once, so make sure not to take more than the recommended number of pills at one time. If you are taking aspirin over the counter, follow the directions on the label. If you are taking aspirin under medical supervision, follow your doctor's instructions. Aspirin may be a widespread, easily available, long-used drug, but it becomes harmful at the wrong dosages. It also interacts with a number of drugs and health conditions.