* Inter: This is the most common opposite, meaning "between" or "among." For example, "intracellular" (within a cell) is the opposite of "intercellular" (between cells).
* Extra: This means "outside" or "beyond." For example, "intracranial" (inside the skull) is the opposite of "extracranial" (outside the skull).
* None: Sometimes there isn't a direct opposite. For example, "intramuscular" (within a muscle) doesn't have a direct opposite.
To find the best opposite, consider the specific word "intra" is modifying.