Well first of all, if you are taking that much
Valium at once, then you obviously have a problem and need help.
As to how much you can OD on, it does vary from person to person and how it affects them, those who take it on a regular basis tend to build up a tolerance to it, and it takes much more for them to have an adverse reaction to it, people who have never taken it before, or stopped taking it for awhile, will suffer adverse effects from a much lower dose, because their bodies aren't used to the drug.
As to long term side effects, you might not feel them, but they are there in your body. Taking pills like that can affect your stomach, liver, kidneys and etc. You might not have symptoms from that damage right now, but if you continue on this path, you will eventually get hit with them.
An overdose usually occurs when you get into thousands of MGS of a pill, because then you are breaking into grams of a med, instead of mgs, 1000mgs equals a gram.
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My information is not guaranteed correct. I do not get them right all the time, but I do enjoy the hunt~