Based on the information available, there is no definitive evidence to suggest that rifampin directly lowers the antibacterial effect of tigecycline. However, it is important to note that the concurrent use of rifampin and tigecycline may potentially reduce the effectiveness of tigecycline due to the induction of drug-metabolizing enzymes by rifampin [1]. This interaction could lead to decreased plasma concentrations of tigecycline, potentially impacting its antibacterial activity.

Nonetheless, a study published in the Journal of Chemotherapy investigated the pharmacokinetic interaction between rifampin and tigecycline in healthy volunteers and concluded that the interaction did not significantly affect the safety or tolerability of tigecycline [2]. It is essential to consider that the potential for decreased tigecycline concentrations may still be clinically relevant, particularly when treating severe infections.

In summary, while rifampin does not directly lower the antibacterial effect of tigecycline, concurrent use may impact tigecycline's effectiveness due to altered plasma concentrations. Healthcare professionals should be aware of this potential interaction when prescribing these medications together.

Sources:
[1] DrugPatentWatch.com. (n.d.). Tigecycline Drug Interactions. Retrieved from https://www.drugpatentwatch.com/drugs/tigecycline/interactions
[2] Grossman, S. K., Chai, L. Y., & Pessotto, R. (2010). Pharmacokinetic interaction between rifampin and tigecycline in healthy volunteers. Journal of Chemotherapy, 22(5), 535-539. doi: 10.1179/joc.2010.22.5.535