NO way to tell. we don't know what country your even in.
How many phentermine. 10 or 100.
What mood is the customs agent in that day? Seriously if you had to show up to work everyday, in a dusty garage on the side of an airport. Constantly passing packages through monotonously, how hard would you look.
Did you send it? Or is it from a IOP? If an IOP using similar packages too much, a customs agent might just see it as familiar and put it aside for search.
All factors, some of which are impossible to determine.
I've had packages opened at US customs that were obvioulsy pills, and they just sent it through. I've had stuff nicely packaged get seized. If I was the agent, I probably wouldn't be seizing to many, but maybe this guy just had an espresso and is ready for work! Millions of incedental things affect it. How fast is the mailer this week. Does your package arrive and sit in time to get checked by me? Or does it come through on the next shift where the go-getter is working?