Very common (10% or more): Amenorrhea (up to 68%), bleeding (up to %), uterine bleeding irregularities (up to 35%)
Common (1% to 10%): Dysmenorrhea, leukorrhea, vaginitis, intermenstrual bleeding, urinary tract infection, vaginal candidiasis, vaginitis, vaginitis bacterial, abnormal cervix smear, metrorrhagia, menometrorrhagia, menstruation irregular, vaginal hemorrhage, erectile dysfunction, genitourinary tract infection, pelvic pain, dyspareunia
Frequency not reported: Uterine cervical erosions, cervical discharge, vulvovaginal dryness, premenstrual syndrome, vaginal cyst, ovarian cyst, lack of return to fertility, sensation of pregnancy
Postmarketing reports: Unexpected pregnancy, uterine hyperplasia, oligomenorrhea, prolonged anovulation [ Ref ]
The structure of cyclocreatine is fairly flat (planar), which aids in passive diffusion across membranes. It has been used with success in an animal study, where mice suffered from a SLC6A8 (creatine transporter at the blood brain barrier) deficiency, which is not responsive to standard creatine supplementation.  This study failed to report increases in creatine stores in the brain, but noted a reduction of mental retardation associated with increased cyclocreatine and phosphorylated cyclocreatine storages.  As demonstrated by this animal study and previous ones, cyclocreatine is bioactive after oral ingestion   and may merely be a creatine mimetic, able to phosphorylate ADP via the creatine kinase system.