Is it safe??

I’m underweight for my age. I’m 17 years old, 5’2” & weighing 82 lbs. my gynecologist advised me if needed, to avoid Plan B & go with an emergency contraceptive called Ella instead.

She didn’t tell me why but I assume it’s because my body is tailored better to the effects of Ella than Plan B.

Knowing this, should I just wait for Ella? Or go against what she advises & get Plan B instead? Would it be safe for me?