Just took my first 5mg dose of Eliquis 3 hours ago. Developed mild headache. Should this be cause for concern? At which point should it be checked?

Headache is not a. common side effect to Eliquis. Perhaps the stress and concern you have with a new medication is enough to give you a mild headache. If it persists for a few more days or worsens then contact your health care provider and explain your concerns.