Say you want to know what the possible effects of taking a supplement might be. You could research the supplement on the two high-quality websites that report supplement interactions based on human trials:
The Mayo Clinic - not as many supplements covered.
It would be great if there were reliable "trip reports" from patients on the effects and side-effects of drugs, but unfortunately side-effects are not reliably reported.
If you wanted more basic information, you could consult a metabolic pathway interaction diagram. Note that the study of genetics and proteomics still has a long way to go: we don't know what most of the essential genes even do, nor do we know the function of xx% of all genes. No network diagram is complete....
SigmaAldrich offers a searchable poster:
According to this, NAC can increase glutathione, but also homocysteine. Important information from the network!
Biochemical-pathways.com has even more information. Note that because the network diagram is again a poster, single compounds (e.g. cysteine) can occur in different places on the diagram.
Metacyc is the most detailed, but only shows one "pathway" at a time.
KEGG is another very good resource with drop-down menus to explore individual pathways.
A long list of other resources.