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.
If any part of your flight is international, especially if the travel requires a visa, expect to go through check-in at the airport as usual. Early check-in may not be available if you have special travel circumstances, such as traveling with a pet or getting a special military fare.