A toothbrush isn't the best prostate massager either. You need to get an anal probe - one that vibrates and the kind you can only get at like an adult store or online.
For me, my comfort zone ends when it comes to putting anything up my keyster. I can have plenty of fun without anything going where it shouldn't and my girl is happy enough with that.
And if you do use lubricant, you should use something water based. The lining of your rectum is is just a thin membrane than easily be torn. With no nerves to registor pain, you may not even notice small tares. But these tares can invite infection - one more reason I'm not into anything going up my second door.