Choosing supplements is confusing.

I stick with brands with a good track record, who label their products with GMP or USP, and who label their products accurately (no nonsense). I avoid "proprietary blends" which are ways to obscure contents. I also avoid shot gun mixtures, because you cannot adjust your doses to your own needs and are forced to use the ratio in that particular brand.

Examples of good companies:
Doctor's Best
NOW
Country Life
Puritan's
LEF ( but expensive and sometimes labeling tricks)
Jarrow
NatureMade
Schiff ( I use their Melatonin)
Nature's Way (good for herbs)
Source Naturals
Solgar (tends to be a bit more expensive, but is quality)

I buy online almost exclusively. I avoid GNC and other shops locally (they tend to be very expensive). You can find some quality brands in some natural food stores like Whole Foods. Online places like iherb.com have detailed supplement ingredients, including inert ingredients, and allergenic labeling. That is one reason I often recommend them. They are affordable and fast too. Amazon has picked up the slack with free shipping on $25 for many supplements now, but they do not offer lots of labeling info which is a negative. You have to KNOW what you want to use Amazon. I am getting my Doctor's Best products from them now mostly (3 things I use daily, benfotiamine, RALA, SunTheanine).

example of iherb page:
http://www.iherb.com/Jarrow-Formulas...enges/117?at=0

I find quality at Costco, and Sam's club.

The trickiest thing is understanding the newer forms of some vitamins and supplements. Lipoic acid and B12 have new data on them, and hence not all companies are on that band wagon yet. So for example the R-lipoic acid from Source Naturals is just not as good as Doctor's Best. But Source Naturals is a quality brand anyway, they are just not using the new stabilized form of R-lipoic.

A quality manufacturer also has informative and useful web pages. Not splashy fancy ones. Quality places do NOT make you look for ingredient lists...and are helpful and give their address and/or phone # on them for contact. Places without contact information should be avoided.

But even within a quality place there can be subtle differences.
For example NatureMade l-lysine has a propylene glycol additive in them, which is a laxative and I think should be avoided. Puritan's Pride capsules do not have this in them.
This is a matter of understanding most consumers do not have.

I've been researching things for over a decade in this area, so I have more intuitive responses than others. You can always post, a question and have me look to see what I see there.
That is one reason this board is helpful. Sharing experiences.

Regarding the bathroom question:
Do you have a history of antibiotic use? If so, you may have damaged beneficial bacteria in the bowel, and/or Candida overgrowth. The Candida can upset your metabolism, and send toxins that give PN type symptoms. If you suspect this, try using Kefir daily. This has 12 probiotic organisms in it and may help re-establish normal things for you. Some SSRI antidepressants loosen people too. This is because they stimulate serotonin in the bowel. TCAs like Elavil do not do this and tend to be constipating.
Also it may help to use Metamucil (either powder to mix into liquids or capsules with lots of water). Metamucil will adsorb toxins in the bowel and normalize movements when things are upset. You must space this around RX medications however. If you use a magnesium oral supplement (esp OXIDE form) you may be loose too. Avoid OXIDE.

Kefir:
http://www.lifeway.net/

I started Kefir in September and it has been one of the most valuable things I've done for myself.