Hi, my mum has a shih tzu dog that has dry red itchy skin, apparently common with this breed. She has tried everything now she gets medication from vet. He still scratches but a little less severely. Love some advice

Hopefully the medicine your mum gets is NOT a steroid injection.
The best thing for this issue is a diet that does not contain by-products, corn, BHA,BHA or other preservatives and additives. Read the labels this will rule out 99% of the dog foods at a local petfood store or grocery store. Better diets cost more, but your dog will eat less, feel better, live longer, and you will not have to spend it at the vets office.
Also, the better foods have increased omega 3 & 6 fatty acids, fish oils, and seaweed or kelp for proper thyroid function.
See www.naturapet.com for food choices and comparisons

I see more nonsense about dogs' diets than anything else on the net. I hope your mother doesn't waste any time and money vainly pursuing expensive dog chows that mainly benefit the companies that produce them. The fact is, corn oil is a great source of the linoleic fatty acid dogs need to keep their skin and coats in good shape. All but the cheapest dog chows have plenty of it. The important thing is make sure the dog is getting a complete and balanced diet by feeding it a commercial chow and little else. There are plenty of websites out there shrieking about terrible ingredients in most dog chows. What you will never see is a scientific, controlled study showing corn and byproducts are harmful to dogs. What you will find is millions of dogs thriving on chows containing them. This includes the extremely valuable, hard working service dogs I am very familiar with. I have a young untrained pup and a retired dog in my house. One is eating Pro Plan, and the other Iams.

How often is the dog brushed? A daily brushing with a soft bristled brush and avoiding skin drying baths should help.