Synarel Side Effects: What to Expect and When to Call Your Doctor

Starting Synarel (nafarelin acetate) means your body is going through a significant hormonal change. By design, Synarel causes estrogen (and testosterone) levels to drop dramatically — this is how it treats endometriosis and central precocious puberty. But that hormone drop comes with a predictable set of side effects that every patient and caregiver should know about before starting treatment.

This guide covers what side effects are expected, which ones require a call or visit to your doctor, and how to manage the most common ones.

Why Synarel Causes Side Effects

Synarel is a GnRH agonist. When taken twice daily, it desensitizes the pituitary gland to gonadotropin-releasing hormone, causing LH and FSH to drop. This reduces estrogen production by the ovaries by up to 95%. The result is a state similar to menopause — temporary, but real. Most of Synarel's side effects stem directly from this low-estrogen state.

There is also a brief initial hormone surge in the first few weeks of treatment before suppression takes hold. This can temporarily worsen symptoms before they improve.

Common Side Effects of Synarel (Expected)

These side effects are common and typically expected with Synarel use. Tell your doctor if they're severe or significantly affecting your quality of life:

Hot flashes and night sweats: One of the most common side effects, caused by the drop in estrogen. Dressing in layers and using cooling strategies can help.

Vaginal dryness: A result of low estrogen. Water-based lubricants can provide relief. Do not use oil-based products with latex contraceptives.

Mood changes and emotional lability: Crying, irritability, impatience, and mood swings have been reported. These are related to hormone fluctuations.

Headaches: Mild-to-moderate headaches are reported by many patients. Persistent or severe headaches should always be reported.

Decreased sex drive: Low libido is a common consequence of reduced sex hormones.

Acne: Hormonal fluctuations early in treatment can trigger acne. This often resolves as hormone levels stabilize.

Muscle pain: Aches and myalgia are reported in some patients.

Reduced breast size: Due to low estrogen. Reverses after stopping the medication.

Nasal irritation: Mild discomfort, dryness, or irritation in the nasal passages from the spray itself.

Irregular vaginal bleeding: Light spotting or irregular periods are common in the first 1–2 months as hormone levels adjust. Persistent bleeding after 2 months should be reported.

Serious Side Effects: Call Your Doctor Right Away

These side effects are less common but require immediate medical attention:

Seizures: GnRH agonists have been associated with convulsions. Risk is higher in patients with a history of epilepsy, brain tumors, or who take bupropion or SSRIs. Seek emergency care immediately if a seizure occurs.

Pseudotumor cerebri (increased brain pressure): Reported particularly in children. Symptoms include severe headache, vision changes, ringing in the ears, dizziness, and nausea. This is a medical emergency.

Depression: Feelings of hopelessness, persistent sadness, loss of interest, or thoughts of self-harm require immediate medical attention. GnRH agonists are associated with psychiatric events.

Bone density loss: Extended use of Synarel can reduce bone mineral density. This is typically reversible after stopping, but patients on long-term therapy for CPP should have bone density monitored. Use of add-back therapy (low-dose estrogen/progestin) can reduce this risk for adults.

Severe allergic reaction: Patients allergic to nafarelin or related GnRH analogs (goserelin, leuprolide, histrelin, triptorelin) should not use Synarel. Signs include rash, swelling, difficulty breathing.

Do Synarel Side Effects Go Away?

Most common side effects of Synarel — including hot flashes, vaginal dryness, mood changes, and decreased libido — are reversible and typically resolve after stopping the medication. Hormone levels generally return to normal within 4–8 weeks after discontinuation. Bone density may take longer to recover and may not fully return to baseline in all patients.

Special Note for Children on Synarel

Children using Synarel for CPP may experience an initial worsening of puberty symptoms in the first 4–6 weeks. This is an expected "flare" effect before hormone suppression takes hold. Pubertal signs should not progress after the first month of treatment. If they do, contact your pediatric endocrinologist promptly. Behavior changes and emotional symptoms in children should also be reported to the doctor.

For a full list of drug interactions that can worsen side effects, see our guide on Synarel drug interactions.

For a full overview of how Synarel works and what it treats, read our complete Synarel guide.