When a drug enters the body via oral ingestion, smoking, snorting, or injection, the chemicals move through the bloodstream and must cross what barrier to produce an effect?

Blood Brain Barrier(BBB)

Explanation:

This is a biological interface of special endothelial cells of astrocytes, pericytes , capillaries,  neurons and other  structures, acting as a physical barrier or  gate separating the Central Nervous system from the peripheral  blood circulations. Thus it  formed a tight junction serving as a  physical barrier to  selectively regulate the entrance of substances from the   cerebral blood flow   into the brain,  therefore maintaining homeostasis of the brain.

From above any  substances  absorbed into the blood stream  irrespective of the mode of entry  are transported through  the BBB, and  are selectively  regulated to enter the brain cells by the BBB's   interactions with the blood vessels, hormones etc.

Basically it allows the passage of nutrients,  a few  drug, hormones and resists certain toxins and pathogens for the brain homeostasis.

the correct answer to this question is #5