Entirely depends on the amount of spirits they use, and the size. If you're just wanting to test the water, don't be afraid to ask them to make it weak, there's no shame and no one will judge. And drink slowly.
Decades ago I used to carpool to work. If I wasn't driving, I'd near always fall asleep on the ride. I'd also for some reason near always get wood while nodding off. I don't think anyone ever noticed, or at the very least they were kind enough not to say anything.
Australian.