Given the data you provided, it seems likely that both you and your boyfriend have inflammation and infection caused by the same bacteria. No specific data about treatment of the bacterial vaginosis was provided. The name of the bacteria was not given either. There are some bacteria, like Escherichia coli, that may often cause bacterial vaginosis and are not always considered as reason for STDs. The fact that Flagyl cured the infection which later reappeared, might indicate that reinfection has occurred.

One of the most common mistakes in STIs treatment is treating only one of the two (or more) sexual partners, consequently transmitting the infection during the next intercourse, from the infected partner to the treated partner.

In order to avoid this, treating both (or rather: all partners) at the same time as well as sexual abstinence (or using condom permanently) is recommended. After the treatment, several tests are needed to make sure that the bacteria are eliminated from both/all partners. (If there is third person involved in the sexual relationship of two, you might as well be having a “table-tennis effect”: completely cured, but caught again the infection during the next sexual intercourse and passing it on again).

If tests results suggest existence of bacteria at least in one of the partners, you might want to consult gynaecologist (your boyfriend might want to consult urologist) and eventually repeat the treatment.