Progesterone Side Effects
Progesterone therapy causes common side effects including premenstrual-tension-like symptoms (mood swings, bloating, fluid retention, sleep disturbance), headaches, breast pain, and irregular vaginal bleeding, with serious but less common risks including venous thromboembolism, stroke, and cardiovascular events, particularly when combined with estrogen 1.
Common Side Effects
The side effects of progestational compounds vary with the specific agent administered 2. The most frequently reported common side effects include:
- Premenstrual-tension-like symptoms: mood swings, bloating, fluid retention, and sleep disturbance 2
- Headaches 1
- Breast pain and tenderness 1
- Irregular vaginal bleeding or spotting 1
- Gastrointestinal symptoms: nausea, vomiting, stomach or abdominal cramps 1
- Hair loss 1
- Vaginal yeast infection 1
Switching among various progestational agents may decrease these symptoms if they become problematic 2.
Serious Side Effects
Cardiovascular and Thromboembolic Events
The most concerning serious side effects involve abnormal blood clotting and cardiovascular events 1:
- Venous thromboembolism (VTE): deep vein thrombosis and pulmonary embolism 3, 4, 1
- Stroke 3, 4, 1
- Myocardial infarction (heart attack) 1
- Visual loss or blindness 1
When combined with estrogen therapy, the risk profile changes significantly. In the Women's Health Initiative study, combined estrogen-progestin therapy showed increased risks: 8 additional strokes, 8 more pulmonary emboli, 7 additional coronary heart disease events, and 8 more invasive breast cancers per 10,000 women per year compared to placebo 3.
Cancer Risks
Breast cancer risk increases with combined estrogen-progestin therapy, particularly with longer duration of use 3, 4. The risk appears to increase as a function of treatment duration 5. However, the specific progestogen type matters—some observational data suggest micronized progesterone may have different risk profiles than synthetic progestins 5.
Central Nervous System Effects
Micronized progesterone capsules specifically can cause extreme dizziness and drowsiness, particularly during initial therapy 1. In some cases, symptoms may include:
- Blurred vision
- Difficulty speaking
- Difficulty walking
- Feeling abnormal
These symptoms require immediate discussion with a healthcare provider and caution when driving or operating machinery 1.
Fetal Risks
Progesterone exposure during pregnancy has been associated with congenital anomalies, including cleft palate, cleft lip, hypospadias, ventricular septal defect, patent ductus arteriosus, and other congenital heart defects 1.
Metabolic and Other Effects
Differences exist among various progestational compounds in their effects on 2:
- Lipid profiles (HDL and LDL cholesterol)
- Androgenic potency
- Glucocorticoid and antimineralocorticoid activity
- Procoagulant activity and fibrinogen levels
These differences may have clinical significance affecting insulin sensitivity, glucose tolerance, fluid retention, blood pressure, and vascular dilation 2. Lipid profiles and cardiovascular risk variables should be monitored to determine individual risk 2.
Route-Specific Considerations
Transvaginal or transdermal progesterone may achieve adequate therapeutic effects with fewer systemic side effects compared to oral administration 2. Some women experience unacceptable side effects from all orally administered progestagens and may benefit from alternative routes 2.
Important Contraindications
Progesterone should not be used in patients with 1:
- Peanut allergy (micronized progesterone capsules contain peanut oil)
- Unusual vaginal bleeding
- Current or history of certain cancers (particularly hormone receptor-positive breast cancer)
- Current or history of blood clots
- Current or history of stroke or heart attack
- Liver problems
- Pregnancy (when used for non-pregnancy indications)
Monitoring Recommendations
To minimize serious side effects, patients should 1:
- Report unusual vaginal bleeding immediately
- Have annual pelvic exams, breast exams, and mammograms
- Discuss continuation of therapy regularly with their healthcare provider
- Seek immediate medical attention for warning signs: changes in vision or speech, sudden severe headaches, severe chest or leg pain, shortness of breath, dizziness, or faintness
The FDA recommends using the lowest effective dose for the shortest duration consistent with treatment goals 4.