Is it rare for tooth infection to spread and cause the throat to swell shut? i am on sulfamethoxazole. paranoid.

Not likely: Tooth infection can spread in very sick people, but if your throat is swelling you need to see a doctor asap. There can also be allergic reactions to the antibiotic -- and this would be the more likely cause of throat swelling. Please go to the emergency room if this is happening to you! your prescribing physician also needs to know.

Rare: Dental infections do tend to involve surrounding oral tissues, but severe cases of so-called odontogenic deep neck space infections and cellulitis (e g ludwig's angina) are fortunately uncommon in otherwise healthy folks. Early treatment makes spread of infection less likely, but sulfamethoxazole would not be a first choice. Get rechecked if no improvement, especially for any breathing difficulty.