Thanks for the quick reply.
My name is XXXXX XXXXX I will be very pleased to talk this over with you. While I cannot precisely diagnose your Schnauzer's problem via the internet I can make the following general comments based on my past experience of similar cases over the years :
Drinking excessively and weight loss such as you describe would always be a cause for concern, the chances are some disease process is causing these symptoms. I am so sorry that I cannot diagnose this via the internet, of course guessing is no good to you, it will just cause more confusion.
What I will do though is to talk you through just some of the possible causes of excess drinking [ polydipsia ] that I know. This is the personal list I use when presented with a dog like yours, the chances are that the disease your dog is suffering from is here somewhere.
1. Diabetes mellitus - sugar diabetes - easy diagnosis via blood test
2. Diabetes insipidus - rare but does happen - catches vets out !
3. Kidney problems - diagnosed by blood tests
4. Chronic hepatitis - diagnosed via blood tests
5. Cushings syndrome
Here is a good link on the subject so you can look at things in depth : LINK
Since there is such a range of diagnosis the only advice I can give you is that you should have your dog checked over by your vet again sooner rather than later. There is rarely any scope for home remedies in this sort of situation. A quick blood test will very often shed light on the matter
If I have not covered your question fully enough or you would like to ask more I will be online for the next hour or so and I will be at your disposal.Scott Scott Nimmo BVMS, MRCVS.
I have over twenty five years full time experience in treating domestic pets and am pleased to do my best to advise you with your current problem. However please note that this advice and any subsequent advice I may give is for your interest and education only and is not intended as a substitute for an in-person consultation with a qualified veterinarian.