These terms are often used incorrectly so it’s important to understand the difference. IC50 is the drug concentration causing 50% inhibition of the desired activity. Strictly speaking, IC50 only applies for inhibition of a specified individual target, e.g. inhibition of an isolated enzyme, or reduction in fluorescence for a luciferase reporter assay for an individual protein of interest, etc.
For whole-cell assays, GI50 or ED50 should be used. GI50 is the concentration for 50% of maximal inhibition of cell proliferation, and should be used for cytostatic (as opposed to cytotoxic) agents.
ED50 (or EC50) is the Dose (or Concentration) causing 50% of maximum effect for any measured biological effect of interest. This could include cytotoxicity (although in this case it is better to use LD50 in my opinion). LD50 is the concentration causing 50% cell death (LD = lethal dose), although some people may have the opinion that LD50 should be reserved for in vivo studies only.
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