Some medicines can cause unwanted and even harmful effects if you have certain medical conditions (e.g., diagnosis, contraindication, allergy). For example, taking a medicine to which you have an allergy can result in severe breathing and skin reactions. Or, you may worsen your condition (e.g., high blood pressure) by taking an over-the-counter medicine (e.g., decongestant such as pseudoephedrine). Keep in mind that a medicine's inactive ingredients (e.g., soy, gluten) can also cause reactions with existing medical conditions.