Phentermine - 15 mg vs 37.5 dosage & results. I'm starting in 2 days?

Hi, JM!
Yes, we're here but we're not professionals. We share experience and some knowledge.
The dosage is dependent on several factors, current weight, BMI, medical history, whether you have any concurrent conditions, and so on.
The dosage is going to vary from person to person according to their response to the medication. 15mg is a good starting point... you may find it sufficient but there's a chance you may need to adjust your dosage along the way. 37.5 is a pretty max dose and I think would only be prescribed after titering the dosage up from the lower doses... the aim being to find the lowest possible dose that gets the job done.
Phentermine enters your blood stream and it peaks at about 3-4 hours and some people begin noticing appetite suppression and better energy in that time zone. Be aware though that it could take several weeks before you notice significant weight loss.
If you have more concerns or if I misinterpreted your question feel free to comment below.
Regards and good luck to you!
Wildcat