,It is possible to have a sputum negative result but still prescribed anti tubercular medicines. Fever,cough,loss of weight, or any other finding in blood test
may indicate active tuberculosis
or even some time therapeutic trial may be given in endemic areas.
Your doctor knows best .As far as side effects are concerned watch for any development of nausea vomiting and Yellow colour in eyes that is Jaundice
which may occur in minority of cases where some drugs needs to be stopped. All the best.