First and foremost, mother's milk is best for the baby and exclusive breast feeding should be done till 6 months of age. It is nice to know that you were nursing the baby till one month of age when your baby was found to be underweight
. At this stage two things should have been done. Firstly to assess your milk production and secondly whether your feeding technique was correct.
When there is uncertainly about the amount of milk production, the milk is expressed in a calibrated jar and measured every 2nd hourly. If the milk production is around 25 to 30 ml in the first month it is adequate. The next thing is the correct feeding technique. You need to know that correct latchment is necessary, your baby's position correct and mouth correctly secured for feeds.
Only when you are convinced that milk production is indeed less and latchment is correct should you add supplements. Please do not discontinue nursing your baby. Motivation is the single most important factor in nursing your baby with adequate milk production. Lactare granules do not have have a known mechanism of action but work more on the psyche of the individual. So give top up feeds to your baby along with your milk.