You shouldn't take aspirin if you have either chicken pox or the flu, because of the very rare possibility of getting Reye's syndrome, a toxic reaction to aspirin that affects children almost exclusively but can occasionally affect an adult. In addition, if you have a medical problem such as diabetes, asthma, kidney problems, or blood pressure that is either too high or too low, be sure to talk to your doctor before starting an aspirin regimen. Aspirin seems to lower blood pressure and can inhibit kidney function in individuals with diabetes or kidney problems. It's also possible for aspirin to trigger an asthmatic effect. However, the chances of any problems occurring from taking just one aspirin a day are slim. If you do fall into any of the categories mentioned, talk to your doctor to see if there are possible alternatives.