Is it necessary to finish antibiotics if you feel completely fine?

Follow what you were prescribed, and if you want to stop early, ask rather than decide alone. The traditional 'always finish the course' advice is being refined — for some infections shorter courses are now evidence-based and are prescribed as such from the start. What has not changed is that you should not shorten a course yourself, and that this matters most for infections like TB where incomplete treatment directly drives resistance. The other half of the problem is taking antibiotics that were never needed — for colds, flu and most sore throats they do nothing at all.