Questions about secondary or unwanted effects a drug can cause on a person; ie: how depression medication can cause suicidal thoughts.
Side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended.
While side effect is mostly used to describe negative, adverse effects. Sometimes it can also apply to beneficial, but unintended, consequences of the use of a drug.
Occasionally, drugs are prescribed or procedures performed specifically for their side effects (off-label-use). In that case, said side effect gradually ceases to be a side effect, and becomes an intended effect. For instance, X-rays were historically (and are currently) used as an imaging technique; the discovery of their oncolytic capability led to their employ in radiotherapy (ablation of malignant tumours).