Hi Carol
I presume you know the dose of Doxycycline for treatment of SLO and other immune mediated skin disease is: 5mg/kg/24hrs (Tapp 2002). Niacinamide is given at the same dose whether you are treating with Tetracycline or Doxycycline.
Extract from: Manual of Skin Diseases of the Dog and Cat By Sue Paterson
Dogs<10kg, 250mg three times a day
Dogs >10kg 500mg three times a day
Give three times a day until noticeable nail growth is seen 3-6 months. Then give twice a day for two months, then once a day for maintenance.
Plumb's Veterinary Drug Handbook gives exactly the same dose for niacinamide, however in the book Clinical Immunology of the dog and Cat by Michael J Day it is stated the same dose but only given twice a day.
If adverse effects become a problem then reduce the niacinamide dose first (Plumb's Veterinary Drug Handbook)
Do make sure you are giving niacinamide or nicotinamide, and not nicotinic acid (niacin). Although Niacin acts identically as a vitamin, it increases the blood flow and blood pressure and should not be used.
Doxycycline is the only drug that you need a prescription for and this can be obtained from your vet. You could try Meedicines4pets, Vet medic or Hyperdrug and there are many other online pharmacies, but you could also get a quote from your local pharmacy or from Boots the Chemists if you are in the UK. The other supplements (and niacinamide) can be bought from a health food shop and the oils may be bought cheaper from the supermarket. Natural Vit E is another excellent supplement but it is used in therapeutic doses 400iu twice a day (Clinical Immunology of the Dog and Cat by Michael J Day)
You have to be patient with this treatment but it will pay off in a couple of months.
Jo