For me, a high dosage is important. Usually only the "Exec B" types are strong enough.  Fortunately the pharmacy brands are pretty cheap. I've been taking them over 10 years without problem and am in pretty good shape. 

If you have access to a traveller to the USA, Costco Sams Club etc have huge 300-500 tabs bottles at a fraction of what they cost here. Often come in large pills that can be cut into smaller pieces.

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