Positive TB-test, but negative X-ray exam... 300 pills for 9 months?

I had a BCG as KID, but always had a positive TB-est and negative X-ray... now a Doctor want me to take 300 pills for 9 months. I don't show any symptoms or so. I am really afraid of the second effects of this antibiotic in my body, or suspect a sort of drug testing on me. What do I do??? the drug is ISONIAZID 300mg.. thanks