It depends on the subjects in which you receive a 6. Usually, med schools are most concerned about biology, chemistry, English, and math.
Although, if you do get a lower grade in English you can always complete a language exam such as the TOEFL or IELTS, so I would say your concern is with the other three.
Of course, if the rest of your application looks impressive you'll have nothing to worry about, but considering how competitive medicine is, you should try your best to keep those subjects as high as possible.