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www.naturesbest.co.uk/alpha-lipoic-acid-300mg-p563/?SRC=GOMISC&gclid=EAIaIQobChMIuImNr5mf1QIVz7DtCh1DUgJJEAAYAS I bought the above Alpha Linoic acid for my husband as he has been suffering from what we belive to have been the shingles virus and we were recommended this to help with 'nerve pain'. He takes 600mg per day but if you are on medication for diabetes (he isn't) then you need to check that it won't affect what you are already taking.
I'd always buy organic vitamins and a good quality multi-vitamin may be all that you need to supplement diet. Taking Metaformin can inhibit the absorption of Vitamin B12 so a supplement of that might be beneficial. As for Vitamin D the best way to absorb that is expose your skin to sunlight for around 15 minutes each day
For a general purpose 'vitamin or tonic' that won't interfere with prescribed medications then I would suggest looking for a Homeopathic remedy. Try having a look at the Helios site-if you phone them they are happy to give free phone advice and I have always found them very helpful.