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.
They should do so, yes. So long as you are not arriving on an international flight and then switching airlines after entry, the agent at check-in should tag your bags to be transfered to your next flight, without your having to retrieve the luggage and take them to the next carrier yourself.