All medicines have risks, and each person may have a different reaction to a particular medicine. Most of the time, the drug facts label should indicate whether a medicine is prone to producing an allergic reaction or a particular side effect. If the label indicates this, then be on the lookout for these symptoms. It is always wise, however, to ask a pharmacist about side effects to watch for, especially with a medicine you have not taken before.
Sometimes, we may have used a medicine before without incident, but the next time we use it we experience a side effect. This is because our bodies may change as we age or the formulation of the medicine has changed. You are the expert where your body is concerned. Even if a label indicates you should watch for a particular reaction, your body may have a different reaction.
Whenever you take a medicine, be extra sensitive to how your body feels afterwards. This is why it is recommended to take the smallest dose possible.
You may be interested in these resources:
Medication Management is a Personal Responsibility: http://www.extension.org/pages/Medication_Management_Is_a_Personal_Responsibility
Avoiding Medication Jeopardy: http://www.extension.org/pages/Avoiding_Medication_Jeopardy
