You recently spoke at my school and I have forgotten a piece of information that you spoke to us about which I would like to know. How many hours does it take until a girl can drive after going out the night before if she is on her P-plates?Thanks for the question … Without a doubt being aware of how long you have to wait before driving after drinking is incredibly important for P-plate drivers. Even full licence holders get this wrong and then end up with a criminal record for drink driving and they can have a blood alcohol concentration (BAC) of 0.05 - P-platers must have a BAC of 0.00! This makes it so much more difficult to stay under the limit. Women have to be far more careful because they take longer than males to process alcohol (particularly P-platers under the age of 21 years, as their livers do not fully develop until this time) and so should wait for much longer before getting behind the wheel of a car.
In my sessions with Year 12 groups I use an actual case study of a young woman who has gone out and had a 'big night' (she drinks 9 standard drinks over a 4 hour period and registers a BAC reading of 0.24). She goes to bed at 2.00am and she does not return to 0.00 until 16 hours later! Realistically that means that she would not be able to drive the entire next day.
Unfortunately, too many people still follow a 'guide' that was provided to drivers when random breath testing (RBT) was first introduced in Australia, i.e., males can have 2 drinks in the first hour and then one drink every hour after that, and for females, one drink in the first hour, etc. Too many people see this as a 'rule', which it isn't - it is a guide, and a very rough one at that. It is based on evidence that suggests the liver gets rid of around one standard drink every hour. It's also assuming that we are dealing with an adult male with a fully-developed, healthy liver - it certainly doesn't apply to the majority of P-platers.
My advice to P-platers is never look at drinking in terms of numbers as it can too easily confuse you and in reality there is no special formula you can use to calculate how long you should wait. Therefore, it is much better to think of it as the type of night you're having (i.e., a 'big night') - do it by the number of drinks and there are too many things to take into consideration…
Therefore my advice for a female P-plater (who must have a BAC of 0.00) who has had a 'big night out' (whatever that may be for the individual) is to wait 24 hours from when she goes to bed (basically don’t drive the next day!) to be absolutely sure that she is under the legal limit. The case study above shows that it took the 19-year-old woman 16 hours to get back to 0.00 – I tell young women to wait longer (just to be sure) as they have additional issues when it comes to processing alcohol, including their menstrual cycle. There will certainly be some young women who will process alcohol much faster but as I always say, it is much better to be safe than sorry…
If you are just having one or two standard drinks, not bottles or cans (which may be far more actual standard drinks), my advice for a young woman is to wait at least 12 hours from going to bed after drinking (once again, just to be absolutely sure!). Driving to a part-time job or to a sporting match the morning after having just a couple of drinks could get you into trouble.
By the time they are 19-years-old, young men have a fully developed liver and so my simple guide for them is if they have had a 'big night out', they should go to bed and wait 12 hours from when they go to sleep. It is important to note though, that if a young male P-plater has made themselves sick or feel unwell in some way due to their drinking, they need to wait longer and, once again, to be absolutely certain that you won't get booked, I would recommend avoiding driving the next day. It's just not worth the risk!
First published: July 2016
Updated: April 2019