A tox screen is a set of standard blood tests for the most common poisons, pharmaceuticals and illegal recreational drugs. When a patient shows symptoms that indicate poisoning or overdose, a tox screen is usually the first diagnostic test ordered by the physician. However, it does have some weaknesses: - It takes time to process the results, although the test is designed to be completed as quickly as possible. - It does not test for common toxins that take time to test for.