Not advisable to give formula milk to a 5 weeks baby, especially if you have enough milk supply. The nipple condition can be treated with proper consultation, other option is you can express breast milk and give it to the baby. Starting baby on formula won't be considered as weaning, it is a substitute to breast milk and is started in conditions where breast milk is insufficient or simply not available. Weaning with solid foods can be started after 4 to 5 months. So you may start the baby on formula in case it is impossible to breast feed her but as i mentioned above that won't be the best thing to do.