Ask a Dentist. Get Answers to Your Dental Questions.
Your dentist is correct, the antibiotic is really only to help keep the infection from spreading in addition to helping get bacterial count down around the tooth/and that will help some with the pain. A root canal is needed when a tooth does a couple things, the nerve/pulp area inside the tooth was injured or diseased (variety of conditions can cause IRP) and the nerve is irreversibly affected and the other is the nerve has simply died on the inside of the tooth (again could die from a variety of reasons). When the tooth is diagnosed as one of these things (irreversible pulpitis or necrotic) then a root canal must be completed in order to removed the diseased or dying tissue (nerve, blood vessel, bacteria) from the inside of the tooth. When taking an antibiotic it certainly helps the tissue around the tooth stabilized from the internal infection, but the antibiotic has no way to get inside the tooth to stop or reduce the bacteria taking over. The only in and out, is the apex of the tooth for these nerve and blood vessel components and if nerve is already dying or dead, they blood vessels aren't functioning to go into the tooth. So scientifically its a lot more than this brief generalization from hopefully it helps you understand why antibiotic won't fix a tooth in the long run when it needs a root canal. Antibiotics can resolve gum tissue issues that arise from periodontal infections because antibiotics can systemically get to those areas. If a root canal wasn't a treatment option you want to proceed with the other route would be just to remove the tooth.
Is there anything specific in addition that you'd like answered?