Drug-drug interactions occur when a drug interacts, or interferes, with another drug. This can alter the way one or both of the drugs act in the body, or cause unexpected side effects. The drugs involved can be prescription medications, over-the-counter medicines and even vitamins and herbal products. Some of the more common symptoms of drug-drug interactions include nausea or stomach upset, headache, heartburn and dizziness. However, if you experience any reaction - after taking prescription or over-the-counter medicine - that seems out of the ordinary, you should consult your local pharmacist and make him or her aware of all of the medicines you're taking. In the case of a serious reaction, you should not hesitate to seek medical treatment. Drug-drug interactions can have serious consequences.