Aspirin can cause false negatives in drug tests. During the study, urine samples from 100 people were tested. Half of the samples that contained aspirin metabolites showed false-negative results on a drug test. The direct addition of aspirin to urine samples did not produce the same results. So, this study confirms that taking aspirin several hours before the test can cause a false negative. Please remember that you need to consult your physician before taking any medications.