My dog won't eat his regular dry food, in 2 weeks all I have been able to get down him is 1 can of dog food, he still drinks plenty of water but is getting weaker by the day, he will eat his dog treats but that is it, he is constantly out of breath and hacks up phlem, I give him antibiotics but that is mostly useless because he just hacks it back up. He is a large havawort 103lbs, and has never been sick in 10 years. Can anyone help me?

There are a number of things that could be causing this. We could take a guess, but it would be just that, a guess. 10 years is good age for a larger breed. I think it's time to go back to the vet.

Where did you get the antibiotics? If they were not given to you by your vet, you should NEVER give an animal antibiotics.

This is something that cannot be diagnosed over the internet. You need to go to the vet, yesterday.

I have lost two large breed dogs in the last few years, one was 12 years and the other just barely 4 years old. Both were victims of cancer, one of the liver, the other of the kidney. Just a gradual going downhill were the first signs, and they went on for several months. After several failed treatments, we did lab work, took x-rays and ultrasounds before finding sufficient evidence of the problems.

I have no clear signs suggesting this as an answer for you, just experience. The age of an animal (and person) dictates which problems may be encountered in a search for a mystery problem. Delaying the search for an answer seldom changes the outcome, but will relieve the constant worry of 'what are we missing?' If possible, find out what the problem is... if it's correctable, do it. If not, the better you can enjoy the time remaining.

Your dog is very sick. Take him to the vet immediately. The respiratory issue sounds like it may be cardiac problems and if so, the lack of nutrition will make him much sicker. He deserves prompt medical care after giving you 10 years of unconditional love.