Keep in mind there is a reason for an expiration date on the bottle of medication; you can check with a QUALIFED pharmacist. First, you should see your physician about your situation. Remember, nothing can replace a PHYSICIAN to get a proper diagnosis and treatment.
The antibiotic properties of the amoxicillan will probably be unchanged and it will be just as useful. On the other hand you should not. I say again. SHOULD NOT be taking antibiotics for a dental infection except under the supervision of a dentist or physician or, in special cases, if you really know what you are doing. First of all NEVER take "just a few". You will end up with what is called an anti-biotic resistant infection. Surprise. People die from such infections. Another surprise. Such infections from the teeth sometimes migrate to the bloodstream and settle on a heart valve. In such a case the valve throws off debri which causes strokes, blindness and paralysis unless you get a pig-valve replacement. If you have another kind of infection it may be viral, in which case antibiotics won't help at all. NEVER EVER take "just a few" antibiotic capsules. You should find a clinic. In my area they treat people without insurance for a flat fee of ten dollars and provide excellent care.
I would not recommend taking any outdated medication. You can check with a pharmacist about the amoxicillin. Secondly, you can ask your physician for samples if possible or ask if she or he can enroll you in a free medication program so that you are able to get the medication to take.
Good Luck!