The kinetics of simulated low-energy daylight (UVA-vis) and powerful combined ultraviolet B and A (UVB-UVA) induced direct and indirect phototransformations of four pharmaceuticals, i.e., ibuprofen, metoprolol, carbamazepine, and warfarin, which were investigated in dilute solutions of pure laboratory and natural humic waters. The results strengthen the essential function of natural chromophores in dissolved organic material (CDOM) as principal photosensitizer toward indirect phototransformations of pharmaceuticals in natural conditions under available low-energy UVA-vis and slight UVB radiations. The results confirmed that organic micropollutants are able to undergo a direct photolysis if their absorbance spectra overlap the spectral range of the available radiation but only if the radiation is strong enough, e.g., ibuprofen is able to undergo only indirect photolysis via different pathways in all realistic conditions. The action of nitrate anions as photosensitizers in the applied conditions proved to be of little importance. High-performance size-exclusion chromatographic experiments verified that the rate constants obtained under the low-energy UVA-vis and powerful UVB-UVA irradiations for the decreased amounts of the two largest molecular size fractions of CDOM were quite close to the rate constants detected for the increased amounts of the next five molecular size fractions with smaller molecular sizes. The decreased contents of the two largest molecular size fractions correlated quite well with the decreased contents of the studied pharmaceuticals under the low-energy UVA-vis irradiation process but somewhat less under the powerful UVB-UVA irradiation. The photochemically induced decomposition of the CDOM aggregates appears to increase the amounts of smaller molecular size fractions and simultaneously produce via CDOM-stimulated radical reactions indirect structural transformations of pharmaceuticals. Apparent quantum yields were estimated for the transformation-degradation of the two largest molecular-size CDOM aggregates under low-energy UVA-vis and powerful UVB-UVA irradiations.