Does cephalexin 500 work for pain in tooth?

Yes it can -: However, by the sound of your question it seem as if you are self prescribing (either have some left over from a previous prescription for something else, or you have a friend who has some left over) concerns are expiration of medicines, taking an insufficient course, and also improper antibiotic. Best to have a consult with dentist and the source of the problem remedied and an rx directly.

If infected..: An antibiotic will work on the infection, but not fix the tooth. See a local dentist for treatment.

Maybe temporarily...: If you have an infection, antibiotics can help control it while you eliminate the cause and/or heal. Like a splinter in your finger, dental pain exists because there is a problem. Would you take pain medication and antibiotics to manage the splinter? Not likely: you would take it out! see your dentist. Solve the problem. Take fewer meds & be well :-).