The new diterpenoids terracinolides J-L (1-3), 13alpha-OH terracinolide F (8), abeodendroidin F (11) and epiabeodendroidin F (12) have been identified from Euphorbia dendroides L. The new compounds and six co-occurring known terracinolides were tested as inhibitors of the drug-efflux activity of P-glycoprotein from cancer cells. The results were used to extend the structure-activity relationships established for this class of compounds highlighting the relevance of substitution at positions 2, 3, 6, and 15 and disclosing a remarkable tolerance toward connectivity changes in the terpenoid core.