There is an excellent article on the topic in Nature: https://www.nature.com/articles/s41526-019-0076-1
So far, the issue of medication shelflife has been dealt with by re-supply, so the study of radiation stability has been “back-shelved” (pun intentional). There is certainly concern for reduced potency as well as toxic byproducts in drugs which have undergone radiation-induced degradation reactions.
The most concerning radiation is charged particles because they have high linear energy transfer, LET, (energy deposited per path length).
In spacecraft outside LEO, the majority of charged particles are protons and alpha particles. But heavier charged particles from Galactic Cosmic Rays (GCR), although less abundant, have much higher LET. As well, they have high hull penetration and spallate into numerous lower atomic weight particles. The spectrum of heavier GCR particles can only be reproduced by the GCR simulator at the NASA Space Radiation Laboratory (NSRL).
Most radiation studies have been done on biological targets, which are mostly water. This research projects poorly onto pharmaceuticals which are often stored dry.
Radiosterilization is commonly used in the pharmaceutical industry. However, a radiosterilized drug is classified as a “new” product and must be re-approved. It is assumed that if a pharmaceutical is stable under high sterilization dose, lower dose over protracted time course is likely safe. However, there is some evidence from NSRL which contradicts this.
Sensitivity to radiation is drug-specific. For instance, Cephradine and cefotaxime, both solid-form cephalosporin antibiotics of similar molecular structure, demonstrate significantly different radiosensitivity. Cephradine degrades significantly and has been determined to be unstable under irradiation whereas cefotaxime demonstrates high resistance and stability.
Another concern is that radiated packaging material could produce additional progeny ions that alter the chemical composition of pharmaceuticals within.
Experience with “flown drugs” returning from the ISS have shown that amoxicillin-clavulanate, levofloxacin, trimethoprim, sulfamethoxazole, furosemide, and levothyroxine degraded before their expiration dates. Impurities were found in aspirin, ibuprofen, loratadine, modafinil, and zolpidem.
Suggested (but inadequately studied) mitigation strategies include cryo storage, radiation-stable packaging, space-hardy excipients and on-board shielding.
Bottom line: radiation effects on pharmaceuticals are a valid concern. More study is needed in anticipation of long space flights outside LEO.