Up to 10 percent of women do not respond to standard treatment. For those who fail treatment, a longer course of metronidazole is recommended by the Centers for Disease Control and Prevention (CDC). If this second course of medication fails, the CDC recommends two grams daily for five days of either metronidazole or tinidazole.If you develop a rash in response to the 5-nitroimidazole drugs, there are other medications you can try, but they have poor cure rates. If you are allergic to tinidazole and metronidazole, your provider may suggest desensitizing you to these medications so you can undergo treatment.