How Does Fostair Work in the Body
Chemical Composition, Mechanism of Action & Metabolic Effects Explained
Key Takeaways: How Fostair Works
- Active ingredients: Beclometasone dipropionate (corticosteroid) and formoterol fumarate dihydrate (long‑acting beta₂‑agonist).
- Primary actions: Beclometasone reduces airway inflammation; formoterol relaxes bronchial smooth muscle for 12‑hour bronchodilation.
- Onset & duration: Formoterol begins working within 1–3 minutes; beclometasone’s full effect builds over days to weeks.
- Metabolism: Beclometasone is hydrolysed to its active metabolite (B17MP) and further metabolised by CYP3A4; formoterol is inactivated via glucuronidation and CYP2D6/2C19.
- Usage: Licensed for asthma and severe COPD in adults; can be used as fixed‑dose maintenance or as part of MART (Maintenance And Reliever Therapy).
Fostair combines an inhaled corticosteroid and a rapid‑onset long‑acting bronchodilator to control asthma and COPD by targeting both inflammation and airway constriction. This dual action helps prevent symptoms and reduce exacerbations.
Important Medical Advice
If you experience sudden worsening of breathing or wheezing immediately after using Fostair (paradoxical bronchospasm), STOP using the inhaler and use your blue reliever inhaler straightaway. Contact your doctor immediately. Also seek urgent help if you develop swelling of the face, lips or throat (possible allergic reaction).
Chemical Composition & Molecular Structure
Fostair 100/6 pressurised inhalation solution contains two active substances in each metered dose: 100 micrograms of beclometasone dipropionate and 6 micrograms of formoterol fumarate dihydrate. The delivered dose from the mouthpiece is 84.6 µg / 5.0 µg respectively. Excipients are ethanol anhydrous, hydrochloric acid and the propellant norflurane (HFA‑134a).
Structural Details
9‑chloro‑11β‑hydroxy‑16β‑methyl‑3,20‑dioxopregna‑1,4‑dien‑17‑yl dipropionate
A prodrug corticosteroid; after inhalation it is rapidly hydrolysed by esterases to the active metabolite beclometasone‑17‑monopropionate (B17MP), which has high affinity for the glucocorticoid receptor.
N‑[2‑hydroxy‑5‑[1‑hydroxy‑2‑[1‑(4‑methoxyphenyl)propan‑2‑ylamino]ethyl]phenyl]formamide fumarate dihydrate
A long‑acting beta₂‑agonist with a formamide group contributing to high potency and duration; the fumarate salt improves stability.
Key Pharmaceutical Properties
| Property | Beclometasone (active metabolite) | Formoterol |
|---|---|---|
| Lipophilicity (logP) | ~3.0 (B17MP) | 2.0 |
| Protein binding | 87% | 61–64% |
| Oral bioavailability | Negligible (extensive first‑pass) | Low (inhaled dose mostly swallowed) |
| Receptor affinity | High (glucocorticoid) | High (beta₂‑adrenoceptor) |
🗒️ Pharmaceutical insight: The HFA‑134a propellant creates an aerosol with fine particles optimised for lung deposition, while the small amount of ethanol helps solubilise the drugs.
Mechanism of Action: Dual Fostair Pathway
Fostair’s therapeutic effect arises from complementary actions on two key aspects of airway disease.
- Beclometasone (corticosteroid): After inhalation and hydrolysis to B17MP, it binds to cytoplasmic glucocorticoid receptors. The ligand‑receptor complex translocates to the nucleus, where it suppresses pro‑inflammatory gene transcription (cytokines, chemokines) and activates anti‑inflammatory genes (e.g., lipocortin‑1). This reduces eosinophil infiltration, mucus secretion and vascular permeability.
- Formoterol (LABA): Formoterol selectively stimulates beta₂‑adrenoceptors on airway smooth muscle, activating adenylate cyclase and increasing intracellular cAMP. This leads to protein kinase A activation, which relaxes muscle fibres and provides bronchodilation.
| Feature | Beclometasone | Formoterol |
|---|---|---|
| Onset of action | Hours to days (genomic) | 1–3 minutes |
| Peak effect | 2–4 weeks | 1–2 hours |
| Duration | >12 hours (receptor occupancy) | 12 hours |
🗒️ Physiological insight: The rapid onset of formoterol allows it to be used as a reliever in the MART regimen, while beclometasone provides sustained background control.
Absorption & Distribution (Pharmacokinetics)
Following inhalation, approximately 30% of the delivered dose reaches the lower airways; the remainder is deposited in the oropharynx and swallowed.
Pulmonary absorption
Formoterol is absorbed rapidly from the lungs, reaching peak plasma concentration within 20–30 minutes. Beclometasone dipropionate is hydrolysed locally and its active metabolite B17MP appears in plasma with a Tmax of about 1‑2 hours.
Distribution
B17MP has a volume of distribution of ~20 L/kg, indicating extensive tissue distribution. Formoterol’s Vd is approximately 4 L/kg. Both cross the placenta in negligible amounts and are likely excreted in breast milk in very low quantities.
Metabolic Effects & Elimination
Beclometasone dipropionate: Undergoes rapid hydrolysis via esterases to the active metabolite B17MP, which is further metabolised in the liver by CYP3A4 to hydroxylated and conjugated inactive products. Excretion is mainly via faeces (60%) and urine (40%).
Formoterol: Metabolised primarily by direct glucuronidation and O‑demethylation (CYP2D6, CYP2C19). Inactive metabolites are excreted renally (~50%) and in bile.
⚠️ Metabolic caution: High doses or hepatic impairment may increase systemic exposure. Beta₂‑agonists can cause transient hypokalaemia and hyperglycaemia; monitor in severe asthma or concomitant diuretic use.
Clinical Efficacy in Asthma & COPD Management
Fostair is indicated for adults (≥18 years) with asthma not adequately controlled on inhaled corticosteroids and as‑needed SABA, or for whom combination therapy is appropriate. It is also licensed for symptomatic treatment of severe COPD (FEV₁ <50% predicted) with a history of exacerbations.
Two usage patterns exist:
- Fixed‑dose maintenance with a separate reliever inhaler (usual dose 1‑2 puffs twice daily, max 4 puffs/day).
- MART (Maintenance And Reliever Therapy): one puff twice daily plus additional puffs as needed for symptom relief (max 8 puffs/day). This approach reduces severe exacerbations by ~25% compared to fixed‑dose regimens.
Clinical trials demonstrate significant improvements in FEV₁, reduced night‑time awakenings, and decreased use of rescue medication. In COPD, Fostair improves lung function and health‑related quality of life.
Fostair FAQs
How long does Fostair take to start working?
Formoterol begins to relax airways within 1–3 minutes, providing rapid symptom relief. The full anti‑inflammatory effect of beclometasone develops over days to weeks of regular use.
Can Fostair be used as a reliever inhaler?
Yes, if your doctor has prescribed the MART regimen. In that case, you use Fostair every day and also take extra puffs when you experience symptoms. Do not use it as a reliever unless specifically instructed.
What are the most common side effects of Fostair?
Common side effects include oral thrush (if mouth not rinsed), hoarseness, sore throat, headache, palpitations and tremor. These are usually mild and transient.
Is Fostair safe for children?
Fostair is not licensed for children and adolescents under 18 years, as safety and efficacy in this age group have not been established.
Can I use Fostair if I am pregnant or breastfeeding?
Fostair should only be used during pregnancy if clearly necessary and prescribed by your doctor. Limited data suggest low risk, but poorly controlled asthma poses greater danger. It is probably safe during breastfeeding, but discuss with your doctor.
Need Fostair with Expert Guidance?
If you have asthma or COPD and think Fostair may be suitable, a UK‑registered doctor can assess your symptoms and provide a prescription online.
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Clinical References & Sources
Medical Content Manager
Nabeel is a co-founder, and medical content manager of Chemist Doctor. He works closely with our medical team to ensure the information is accurate and up-to-date.
Medical Doctor
Dr. Feroz is a GMC-registered doctor and a medical reviewer at Chemist Doctor. He oversees acute condition and urgent care guidance.
Medical Director
Usman is a co-founder, and medical director of Chemist Doctor. He leads the organisation's strategic vision, bridging clinical and operational priorities.
Review Date: 8 March 2026
Next Review: 8 September 2026
Published on: 8 March 2026
Last Updated: 8 March 2026