If azithromycin 250 is expired on 10/20 can it still be used to treat chlamydia. i have 4 pills to make it a two day dose instead of a one time dose. would it still be effective?

It will still be effective. Antibiotics retain their activity, with no increased risk of side effects, for several years after the official expiration date -- assuming they have been normally stored (e.g. not exposed to extreme heat, like in a car's glove box). However, you need to take all 4 pills at once, 1 g total. It may be less effective as 2 pills on each of 2 days.