If you are referring to bacteria, the word "resistant" is applicable, as bacteria adapt to antibiotics by undergoing natural selection to enhance antibiotic-resistant genes.
If you are referring to humans or higher-order species, "grown tolerant" could be used to say that perhaps a certain antibiotic dosage is no longer as effective as it used to be and that a higher dosage is needed. "Acclimated" can also be used to say that an organism's body had to adjust to the exposure to a new, unfamiliar antibiotic.