Alcohol and drugs can have a negative effect on your driving ability
While driving a taxi your Blood Alcohol Content limit is 0% meaning you cannot consume any alcohol before or during your shift. Also alcohol can take a considerable amount of time to leave your body, so consuming excessive amounts of alcohol the day before a shift may still register.
Consuming any form of illegal drugs is prohibited for any driver. However some legal prescription drugs may also have an effect on your ability to drive. Its best to check with your doctor or pharmacist as to whether you are still able to drive.