Impossible? No. Much more difficult after you get a few sessions in? Yes.
It's one of the coolest drugs I've ever taken to be honest. I turn a slight red color from a prescription med I'm on even though I have had a tan for years. At 300mcg ED for a week, the red is gone and I'm easily two shades darker - with the same tanning protocol I've had.
Just be aware of two things:
1. It is an aphrodisiac. You may find yourself stretching in the middle of the day, and BAM! boner city. The random wood goes away within a week or two, just be prepared lol.
2. Moles or freckles also will darken. It impacts all melanin, which is what makes them dark. I found that I actually have a few freckles I never knew about as they were so light.
My .02c