Examples:

• After eating that questionable seafood, I felt sick as a dog all night.
• Poor Max has been sick as a dog with the flu for days now.
• I don't know what he ate, but he came home from the party last night sick as a dog.
• She looked pale and exhausted, sick as a dog from working overtime all week.
• I don't think I can make it to the meeting today; I woke up feeling as sick as a dog.
• After getting food poisoning, he spent the entire weekend sick as a dog in bed.
• I've never seen him this ill before; he's been sick as a dog since yesterday morning.
• Despite being sick as a dog, she insisted on finishing her work before taking a break.
• The poor puppy was sick as a dog after eating that chocolate; we had to rush him to the vet.
• I caught a nasty stomach bug and was sick as a dog for the entire weekend.