What can i do if my girlfriend has missed her menstruation for 4 months, will this have a health effect on me?

No, it shouldn't: Missed/late period: low body wt, obesity, weight ^ or v, ^exercise, endurance athletics, breast-feeding, stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, perimenopause, just starting menstruation, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (i.e., polycystic ovary syndrome). #1 reason is pregnancy.