From the faqs
NSman whose medical leave period, based on a
single medical certificate (MC), exceeds 25% of the duration of his
national service training (excluding courses) may be
deferred. Those whose MCs do not exceed the 25% cut-off are to
report for the training on the day after the expiry of their MCs.
Units are to arrange medical reviews for NSmen who have been
deferred on grounds of medical unfitness on 3 consecutive
occasions. For courses, the decision to take NSman out of the
course on medical grounds is dependent on whether his prolonged
absence will result in him missing a substantial portion of the
course, which cannot be made up. The CO of the training school will
decide on such cases
Does Medical Excuses from Govt hospital also fit in as MC?