Does Phentermine For Weight Loss Affect The Results Of St Johns Herb?

Hello, i am quitting smoking and i want to try st johns wort to help with my nerves and anxiety i had before smoking and hoping it will calm things down when i quit tomorrow. i was wanting to try the st johns herb tonight but i dont know the best ways to take the dosages. does it have to start in the morning? i also took phentermine this morning for weight loss. will that affect the st johns herb?