Disability, certain illness, allergies and individual differences can affect the way people react to medications.Some medications can cause serious problems. Your client may react to medications in a way that is not expected and is dangerous. This is often called an adverse medication reaction. Serious reactions include: choking, changes to or no breathing, changes to or no pulse, loss of consciousness, slurred speech, fitting, bleeding, blurred vision, confusion, loss of balance, swelling, inflammation, rash, vomiting. Your client is at increased risk of other health problems if a medication reaction occurs.