Is it harmful to rely on painkillers for period cramps every month? are painkillers once a month a problem?

Not good idea : It is not a good idea to stay on pain killers all the time ,you need to be evaluated by your obgyn to explore other reasons and also discuss other options

Dysmenorrhea: Motrin like painkillers are common- if no allergy. Opioids are rarely used and not standard . Magnesium 350 mg supplements starting 2 days before the period helps , as does heat , birth control pills as well as thiamine -B1 100 mg, fish oil . B6 helps w/ premenstrual issues. If severe assess cause-fibroids/endometriosis f/u/ w/MD https://www.aafp.org/afp/2014/0301/p341.html