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GOOD BRANDS  a good brand should have
100 mg of B1 (Thiamine)
100 mg of B2 (Riboflavin)
100 mg of B3 (Niacin or Niacin amide)
100 mg of B6 (Pyridoxine)
100 mcg of B12 (Cobalamin)
(as well as other B agents: Folic Acid, Biotin, Choline, etc)

Typically Labeled as: 
"Vitamin B Complex", "B Stress Formula", "B 1000", "B 50", "B 100"

Synthetic Vitamins are cheaper and are just as good as Natural Vitamins (as per Durk Pearson and Sandy Shaw book "Life Extension").

A multi vitamin / mineral is better than nothing. It will keep you alive, having the bare minimum required for life, but most multi-vitamins typically does not have enough of the B Vitamins to keep you Happy.
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ALSO: get an additional B12 1,000 mg (grocery stores) or 2,500-5,000 mcg (vitamin stores)
(I take 25,000 mcg of B12 per day)
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BETTER BUY?   Any grocery store or pharmacy where vitamins are sold. Any brand name. Some may be initially cheaper, but often the dosage is much smaller, thus it may not be the better buy.  You will have to compare and estimate.  60 tabs of 25 mg for $5.50???, or 50 tabs of 100 mg for $11.00??? (the later being the better buy).  
 
INSTRUCTIONS: You can take them with a glass of water (admittedly, they taste kind of "gaggy", but they sure do make you Happy, within 30 Minutes), and you can take them with a meal or on an empty stomach.  (Better to take them with Water than coffee or soda, just to get them down the throat, as coffee and soda are both acidic, and water is just easier). 
IF YOU HAVE NOT HAD B SUPPLEMENTS DAILY for a while, I would advise taking 600 mg per day for at least 2 days, just to get you back in the pink.  WARNING: if you have been low on your B Vitamins and suddenly take a large dosage, you may experience convulsions of laughter (you'll get over it).  
I met an RN (Regestered Nurse) and asked how many milligrams (mg) of B Vitamins she consumed per day, she said 1,000 mg, I laughed and she said "but I need to" (with all the good work she does, good for her). 1,000 mg /day is safe.
TAKE THIS LITTLE LIST TO THE STORE WITH YOU RIGHT NOW