GHB (gamma-hydroxybutyrate) is produced naturally in the human body in very small amounts. When taken as a recreational drug, and especially when taken in combination with alcohol or other drugs, GHB can be extremely dangerous. GHB is a central nervous system depressant.
International flights always allow carry-on luggage, but each airline has its own restrictions as to quantity, size and weight. Some international carry-on policies restrict bag weights to 11 lbs. or less, while domestic flights rarely have a weight limit.
(Hint: when it involves baggage fees.) I fly a lot between the USA and Mexico and each time I need to go through customs and get my passport stamped. According to American Airlines though, flying to Mexico does not constitute an international flight when it comes to one allowed checked bag.