This is why a lot of things used in biological processes need to be purified, one of the steps is separating the enantiomers. Chirality in molecules is common but people might not know what it means, especially when it comes to drugs or supplements. Molecules with chiral centers rotate one way or the other (Dextro- or Levo-rotation) , this can be determined in a lab using light waves, so it's called optical activity. You could find L-carnitine supplements in a grocery store, for example. When synthesized both enantiomers are produced, but only one is useful for biological mechanisms. D-carnitine is considered toxic because it can cause the L-carnitine in your body to be less effective, and you don't want your cell metabolism to stop or slow.

I used carnitine as an example, but stereochemistry is really important to understand when it comes to drug manufacture