Final answer:
Legally entering the U.S. with controlled substances is restricted; marijuana is a Schedule 1 controlled substance federally, despite varying state laws. Prescription drugs and synthetics like fentanyl are also tightly controlled, and unlawful possession carries serious penalties.
Step-by-step explanation:
The legality of entering the United States with controlled substances is governed by the Controlled Substances Act (CSA) of 1970, which makes the sale and possession of marijuana, among other drugs, a prosecutable offense. While certain states have eased laws on marijuana use, it remains a Schedule 1 controlled substance under federal law. Therefore, legally entering the U.S. with controlled substances is highly restricted, and permitted only in very specific circumstances such as having a prescription for certain drugs. Despite state laws legalizing or decriminalizing marijuana for medical or recreational purposes, federal law supersedes state laws, which can lead to prosecution if possession is detected at federal checkpoints or borders. Additionally, opioids like OxyContin and synthetic drugs such as fentanyl are tightly regulated, and unauthorized possession can lead to severe legal consequences.