If you have a medical condition that could endanger you or other passengers on the flight, it is vital that you provide this information to the company with which you are flying. If you have recently undergone surgery or have a severe medical condition, you should not fly. Pregnant women, or those under the influence of drugs or alcohol are also not allowed to fly by most companies. If you have a medical condition that you are unsure if the condition will affect your ability to take a balloon flight, you should check with your physician beforehand. Some companies may require documentation from a doctor that you are fit to participate in a balloon flight and that there is no underlying medical condition that could affect your safety or the well being of the other passengers on the flight.