Funny, I was going to say, look for it on the floor. I would be SO surprised if a cat would eat an antibiotic, especially a giant and not great-tasting honker like cephalexin. He'd be much more likely to bat it off the nightstand and watch it fall, and then look around for something else to knock down.
thank you for the advise was a false alarm I found it under the stand so thank god he didn't get it after all. what a scare!
Call an emergency Vet right now, we are not qualified to answer this....IMO this is a very high toxic dose for a cat and he needs to go to an ER clinic ASAP