What are the common and serious side effects of progesterone in adults?

Progesterone Side Effects

Common Side Effects

Drowsiness and dizziness are the most frequently reported side effects of oral micronized progesterone, occurring particularly after initial dosing, which is why bedtime administration is strongly recommended. 1, 2

Neurological Effects

• Extreme dizziness and drowsiness may occur during initial therapy, sometimes severe enough to prevent operating motor vehicles 1
• Blurred vision, difficulty speaking, difficulty walking, and feeling abnormal can occur in a few cases—these symptoms require immediate discussion with a healthcare provider 1
• Mild and transient drowsiness is the only specific side effect consistently reported with micronized progesterone, minimized by bedtime dosing 3
• Beneficial neurosteroid effects include alleviation of anxiety, depression, and sleep problems when administered orally 2

Gastrointestinal Effects

• Nausea and vomiting 1
• Stomach or abdominal cramps and bloating 1

Gynecological Effects

• Irregular vaginal bleeding or spotting 1
• Breast pain 1
• Vaginal yeast infection 1

Other Common Effects

• Headaches 1
• Hair loss 1
• Fluid retention 1

Serious but Less Common Side Effects

Progesterone carries risks of abnormal blood clotting and fetal harm that require immediate medical attention if warning signs develop. 1

Cardiovascular and Thromboembolic Events

• Stroke 1
• Heart attack 1
• Pulmonary embolus 1
• Visual loss or blindness 1

Warning Signs Requiring Immediate Medical Care

• Changes in vision or speech 1
• Sudden new severe headaches 1
• Severe pains in chest or legs with or without shortness of breath, weakness, and fatigue 1
• Dizziness and faintness 1
• Vomiting 1

Fetal Risks (if used during pregnancy)

• Cleft palate and cleft lip 1
• Hypospadias 1
• Ventricular septal defect 1
• Patent ductus arteriosus 1
• Other congenital heart defects 1

Comparison: Synthetic Progestins vs. Natural Progesterone

Synthetic progestins (Provera, PremPro, Cycrin) produce significantly more side effects than natural micronized progesterone, including metabolic and psychological complications. 4

Synthetic Progestin Side Effects

• Fatigue 4
• Fluid retention 4
• Lipid level alterations 4
• Dysphoria 4
• Hypercoagulant states 4
• Increased androgenicity 4
• Severe psychological side effects 5
• Negative mood effects due to effects on neurotransmitters via central nervous system progesterone receptors 6
• Sodium retention affecting the renin-aldosterone system 6
• Adverse effects on skin, lipids, vasculature, and insulin resistance (particularly nor-testosterone-derived progestogens) 6

Natural Micronized Progesterone Advantages

• Milder adverse effects depending on route of administration 4
• Better bioavailability and fewer side effects than natural progesterone 4
• Preserves full potential of progesterone activity without many side effects of progestins 2
• Better safety profile regarding metabolic ailments, breast cancer risk, and venous thromboembolism risk 2
• Well tolerated with only mild and transient drowsiness as the specific side effect 3

Route-Specific Considerations

Oral micronized progesterone undergoes >90% first-pass hepatic metabolism, resulting in unphysiologically high levels of metabolites that cause dizziness and drowsiness. 5

Oral Administration

• Dizziness and drowsiness to the point of preventing motor vehicle operation due to 5-α reduced metabolites 5
• More frequent drowsiness and dizziness compared to other routes, but well tolerated with nocturnal administration 2

Vaginal Administration

• Fewer side effects than oral progesterone or synthetic progestins when used for hormone replacement 5
• Minimizes systemic side effects due to local direct vagina-to-uterus transport 6