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.
Strangely, policies are sometimes more generous for the weight of luggage on international flights (but see, eg, ATA which contrarily has much less liberal policies for international flights). Most flights originating from the US have a 'two piece' limit for luggage - you can check two bags, and you pay extra per bag. This applies whether the flight is domestic within the US or international. Most flights that originate from outside the US have a weight limit for total checked baggage - a miserly 20kg/44lbs if you're traveling in coach class, 30kg/66lbs for business class and 40kg/88lbs for first class.