Hello from JustAnswer. This is Dr. Love and I will be assisting you today.
Both the doxycycline and the benzoyl peroxide are common medicines used for treatment of acne, including cystic acne. The next medicine that is typically considered if the benzoyl peroxide plus doxycycline is not sufficient would be the addition of a topical retinoid, particularly Retin-A (tretinoin) gel.
It also would be an option to use a topical antibiotic to augment the oral antibiotic, such as topical erythromycin or clindamycin.
There also are some treatments that can be considered for each gender (I realize that you would have stated your gender when registering for the site, but the personal information is hidden to maintain privacy). An oral retinoid, isotretinoin, can be used for males. It is generally avoided in females unless there is very strong avoidance of pregnancy. Females can be treated with an anti-androgen, such as spironolactone, or with certain birth control pills that also help to control acne.
Most of these medicines are only available by prescription. Certain retinoids can be obtained over the counter in some countries, some of which are weaker than the Retin-A gel, such as Retinol. Otherwise, you wold need to discuss the other medicines with your doctor.
If I can provide any additional information, please let me know.