My heart raced after a dental anesthetic injection. Is that a side effect?

In most cases it is a temporary effect of the vasoconstrictor (epinephrine) included in the anesthetic, not a dangerous side effect. It usually settles within a few minutes, so breathe deeply and rest. If you have heart disease, high blood pressure, or an overactive thyroid, or have had strong palpitations before, tell us before treatment and we can use an anesthetic with little or no epinephrine. However, if palpitations last more than 30 minutes or come with chest pain or difficulty breathing, tell us immediately.