Generally laboratory grade reagents have not been prepared with human consumption as a consideration, but with regard to their use as a chemical reagent.
For example, $95\%$ ethanol is mostly ethanol and $5\%$ water. This is fine for cleaning equipment, or using as a solvent for a TLC. If you want to use ethanol as a reagent, however, you want to use absolute ethanol (which is much more expensive). Absolute ethanol is NOT safe for human consumption though, as the water has been removed from the $95\%$ (which is the azeotrope, i.e. as far as you can get through distillation of water/ethanol alone) through a process which involves adding additives such as benzene. Benzene won't cause side reactions in the way that water would, and so absolute ethanol is much better for a lab reagent than $95\%$, but worse for consumption.
In the same way, any chemical which is prepared for use as a reagent will contain impurities which don't generally cause side-reactions, while for consumption you want the impurities to be non-toxic.