F1-drivers train a lot to improve their endurance. Jenson Button did triathlons. Nico Rosberg ran a marathon. I'm sure others to various sports as well. The benefit of a drug like EPO is that you can recover faster from training, which means you can train more and still recover. Apart from endurance, drivers need strong neckmuscles (and maybe other muscles as well). That also involves training. So I'm pretty sure that a performance enhancing drug will benefit a drivers' training program. And if they're barely tested, it's also very likely (imo) that at least some of them will use at least some of those drugs.