Brief Answer:
Get investigated, get a prescription from your Doc

Detailed Answer:
Hi.
Thanks for your Query.
XXXXX very short.
I would like to know whether this is a problem of few days-old or a long standing of few months?
Are you a Diabetic? Or suffered some major illness in the beginning of this problem?

Let us see what can cause dryness of mouth and white tongue.
The commonest causes seen in our day-to-day practice are:

Inflammation of the minor salivary glands which keep the mouth wet when you are not eating. The major salivary glands give saliva only when one is eating / chewing.

General conditions : diabetes, kidney problems, dehydration, Sjogren Syndrome ( eyes are also dry), any disorder which indicates your resistance power has been reduced, Bad oral hygiene and so on.

First of all if you are suffering from any such illness , it has to be well under control for the treatment to be effective.

If there are no major illnesses- great.
You can request your Doctor , preferably an ENT Specialist to initiate a treatment of an antibiotic, metronidazole , anti-inflammatory and steroids ( if you are not a diabetic) for 5 days. There may be a positive effect. Oral anti-fungal in tablet and local application form can be taken if there is evidence of oral thrush ( usually not there as there is mouth is not wet enough).
Supplemental dosage of anti-oxidants in the form of Multivitamins , Vitamin A and D in therapeutic dosage do help.

Soft and liquid diet with lots of fluids to avoid the dry mucosa is very important. Avoid too hot and too cold foods and beverages.

Investigations as per your Doctor's advise for the causes I have mentioned above.

I hope this will help you to get a guideline towards diagnosis and treatment - both.
Fell free to ask further if you have any query.

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