http://www.drugs.com/doxycycline.html

There most likely will be a purging phase.

There are 4 main factors that contribute to the development of the acne you see on your face/body: hyperkeratinization (excessive shedding of skin cells within the pore), oil, bacteria and inflammation.

Your Derm has prescribed you an oral antibiotic called doxycycline, which mainly works by killing acne bacteria but also might reduce inflammation (see here).

However...

Combination Therapy: Appropriate for Most Patients With Acne

 

Because multiple factors are involved in the development of acne, experts stress the importance of combination treatment to target as many pathophysiologic factors of acne as possible. The beneficial effects of combining acne agents include targeting different pathophysiologic factors (ie, abnormal desquamation, P acnes proliferation, and inflammation), increasing efficacy, improving the speed of lesion resolution, and minimizing the potential for antibiotic resistance.

 

Combination therapy, including a topical retinoid with either a topical or oral antibiotic and BPO, is now considered the standard of care for patients with both comedonal and inflammatory acne. Clinical studies have consistently shown faster and better clearing of both inflammatory lesions and comedones with such a combination when compared with antimicrobial therapy alone.