What is the difference between an adverse drug reaction and side effect?

Same thing. It's the same thing - just two different terms. Some side effects could be beneficial and so not an "adverse" reaction but the terms refer to the same effect - unintended consequences (good or bad) of a therapy.

Related Questions

What is the main difference between adverse drug reaction and side effect of drug. Explain it.

Adverse drug reactio. Adverse drug reaction refers to harm associated with the use of a drug at a normal dosage during normal use. Side effect refers to effect of a drug which may be beneficial. Read more...