Sometimes it's tough to know the difference! But having a side effect to a medication does not mean you're allergic to it. Side effects are more common, do not involve the body's immune system, are more likely to occur at higher doses, and can often be prevented. Different people may have different side effects from the same medication. Researchers are finding out that genetics may play a role in how people respond to medication. Side effects are usually an extension of the way the medication works in your body. For example, a pill taken to lower blood pressure may lower it too much. This could make someone feel faint when they stand up. In many cases, the higher the dose of the medication, the more likely a side effect will occur. Drug allergies are less common, involve the body's immune system, can occur with any dose of medication, and can be life threatening.