According to Air Travel (IATA) regulations, you are not permitted to give an animal a sedative prior to a trip by aeroplane.
It is therefore not advisable to give your pet any sedative prior to their departure, as if they are not used to it, it may have the opposite effect to those initially envisaged.
In fact, many sedatives cause a lowering of blood pressure. On a flight, the blood pressure drops anyway. In a pressurised aeroplane, atmospheric pressure is comparable to that at an altitude of 2,500m.
The combined effect of sedatives and altitude may prove dangerous to an animal with a heart condition, one who is sick, old, or very anxious, as it can cause the blood pressure to drop drastically. Moreover, a sedative may have a disorientating effect on a semi-awake animal who may "wake up" in an abrupt, stressed manner.
What's more, airline companies may refuse to allow passage of your pet if they consider them in a state not fit for travel, owing to an abusive dose of sedatives.
If the animal is generally scared, anxious or sensitive, the stress caused by air travel can be severe, in which case your vet may recommend alternative products which help to relax the animal without having undesirable side effects.
In addition, to reduce your pet's stress levels including those related to separation anxiety, you could consider putting some clothing that has your smell on it in their cage, helping your pet to feel more at home and reassured.