Sometimes it is absolutely necessary to give a medication which can cause an allergic reaction and you must have emergency treatments like epinephrine and life support equipment ready when you give the medication in order to act quickly if a reaction occurs. Once you have identified the general risk factors for the individual, it is time to consider the medications which are ordered for appropriateness and possible interactions. Some terminology will help you here. Therapeutic effects are the effects we are trying to stimulate to help the client. Side effects are any effects other than the therapeutic effect.
Adverse effects are undesirable effects.
Toxic effects result in poisoning or damage. Teratogenic effects cause abnormal fetal development. Carcinogenic or oncogenic effects cause cancer. Iatrogenic effects are damage or illness conditions caused by the treatment. Idiosyncratic effects are quirky, unexpected, unusual effects.