The goal of this clinical trial is to explore the effectiveness and safety of Lactobacillus johnsonii in improving ovarian function and perimenopausal symptoms in patients with premature ovarian insufficiency (POI). The main questions it aims to answer are: ① To explore the effectiveness and safety of Lactobacillus johnsonii in improving ovarian function and perimenopausal symptoms in patients with POI; ② To explore the potential mechanism of action of Lactobacillus johnsonii in improving ovarian function and perimenopausal symptoms in patients with POI. Participants who meet all the inclusion criteria will be enrolled in the study and randomly assigned in a 1:1 ratio to either the Lactobacillus johnsonii group or the placebo group.

Inclusion Criteria:

1. Age between 20 and 39 years;
2. Irregular menstruation or amenorrhea for > 4 months;
3. At least two serum baseline follicle-stimulating hormone (FSH) levels > 25 IU/L (with an interval of more than 4 weeks between measurements);
4. Willing to voluntarily participate in this study, sign the informed consent form, and comply with the investigation and follow-up;
5. Not having participated in other clinical trials within the past 3 months.

Exclusion Criteria:

1. Use of antibiotics or probiotics, prebiotics, and live bacterial preparations within the past month or during the intervention period;
2. Received hormone replacement therapy within the past 2 months;
3. Chromosomal abnormalities and genetic defects;
4. Iatrogenic premature ovarian insufficiency;
5. Stage IV endometriosis;
6. Uncontrolled endocrine diseases or autoimmune diseases;
7. Organic gastrointestinal diseases;
8. Infectious diseases of the reproductive, urinary, or digestive systems;
9. Severe cardiovascular, respiratory, renal, hematologic, endocrine, neurological, psychiatric, or other systemic diseases that the investigator believes may interfere with the study evaluation;
10. History of venous thromboembolism;
11. Allergic or intolerant to the components of the investigational probiotic product;
12. Allergic to two or more types of food/medications;
13. Refusal to provide written informed consent;
14. Other conditions deemed unsuitable for participation in the study by the investigator.