Alendronic acid (alendronate)

Bisphosphonates
Bisphosphonates
Pregnancy: avoid
Not used in pregnancy; it stays in bone for years, so a possible effect on the fetal skeleton is a concern even after it is stopped.

An oral bisphosphonate that slows bone loss and lowers fracture risk in osteoporosis. It must be taken in a strict way to protect the oesophagus.

Updated Nov 1, 2026

Also called: Alendronate sodium

Quick facts

Group: Bisphosphonates
Target: Farnesyl pyrophosphate synthase (FPPS), an enzyme of the mevalonate pathway in osteoclasts
Used for: Treatment and prevention · Osteoporosis in men
Key effects: Abdominal pain · Constipation · Musculoskeletal pain
Monitor: Serum calcium · Kidney function · New heartburn
ATC: M05BA04

No doses: the clinician sets them by the current guideline.

Mechanism of action

Farnesyl pyrophosphate synthase (FPPS), an enzyme of the mevalonate pathway in osteoclastsAlendronic acid (alendronate)

The figure shows alendronate itself in the active site of human farnesyl pyrophosphate synthase.

Target in 3D: Farnesyl pyrophosphate synthase (FPPS), an enzyme of the mevalonate pathway in osteoclasts. PDB structure 2F92 (RCSB PDB, public domain). View the structure at RCSB PDB

Alendronate is a nitrogen-containing bisphosphonate. It binds the hydroxyapatite of bone at sites of resorption and is taken up by osteoclasts, where it inhibits farnesyl pyrophosphate synthase. Osteoclasts lose activity and die, resorption falls and bone formation gradually exceeds it, so bone density rises and vertebral and hip fractures become less frequent. It does not stop osteoclasts from being recruited, and it stays in the skeleton for years.

Alendronate binds the bone mineral where bone is being resorbed
Osteoclasts take it up as they dissolve the bone
It blocks farnesyl pyrophosphate synthase and the osteoclast stops working
Resorption falls, bone density rises and fractures drop

Indications

Treatment and prevention of postmenopausal osteoporosis Osteoporosis →
Reduction of vertebral fractures in osteoporosis Osteoporotic vertebral compression fracture →
Osteoporosis in men and glucocorticoid-induced osteoporosis
Paget disease of bone Paget disease of bone →

Adverse effects

Common

Abdominal pain, nausea, dyspepsia
Constipation, diarrhoea, bloating
Musculoskeletal pain
Headache
Taste changes

Contraindications

Oesophageal abnormalities that delay emptying, such as stricture or achalasia
Inability to stand or sit upright for at least 30 minutes
Hypocalcaemia
Hypersensitivity to the product
Not recommended with severe kidney impairment

Key interactions

Calcium supplements, antacids and other products with calcium, magnesium, iron or aluminium: They bind alendronate in the gut and block its absorption; other oral medicines wait at least 30 minutes
Aspirin and other NSAIDs: More upper gastrointestinal irritation
Coffee, juice, milk and mineral water: Much less absorption; only plain water is used

What is monitored

Serum calcium and vitamin D before starting
Kidney function
Bone density and fracture risk over time (need to continue after several years)
New heartburn, pain on swallowing or chest pain
Dental health; thigh or groin pain

Administration and safety

Taken first thing in the morning, after getting up, on an empty stomach, with a full glass of plain water
The tablet is swallowed whole; the person stays sitting or standing for at least 30 minutes and does not lie down
Food, drinks and other medicines, vitamins or antacids wait at least 30 minutes
Weekly forms are taken on the same day each week

Pregnancy and breastfeeding

Not used in pregnancy; it stays in bone for years, so a possible effect on the fetal skeleton is a concern even after it is stopped.

Nursing care

Check that the person can stay upright before giving it; if not, it is not given
Teach the full ritual: on waking, empty stomach, plain water, upright, nothing else for 30 minutes
Ask about heartburn, pain on swallowing or chest pain and report them; the drug may need to stop
Check calcium and vitamin D intake and dental health; ask about planned dental extractions
Report new thigh, hip or groin pain

Pharmacokinetics

Absorption: Very poorly absorbed by mouth; food, coffee, juice and minerals cut absorption almost to nothing
Distribution: Binds bone quickly; the rest is not taken up by tissues
Metabolism: Not metabolised
Elimination: The part not stored in bone leaves unchanged in the urine
Half-life: Its terminal half-life in the skeleton is estimated at more than 10 years

Test yourself

Think of the answer, then tap the question to see it.

Which instructions protect the oesophagus and absorption with alendronate?

First thing in the morning on an empty stomach, with plain water, staying upright at least 30 minutes and waiting before food, drink or other medicines.

Why is alendronate not given with calcium or antacids at the same time?

Calcium and other polyvalent cations bind it in the gut and prevent its absorption, which is already very low.

Can you name two conditions in which alendronate is contraindicated?

Any two of: oesophageal strictures or achalasia, inability to stay upright for 30 minutes, hypocalcaemia, hypersensitivity.

Which rare adverse effect is linked to dental extractions during bisphosphonate treatment?

Osteonecrosis of the jaw: exposed bone that does not heal, usually after an extraction or local infection.

Add to my cards (4)

Connections

← Back to the class: Bisphosphonates

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Alendronate: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); DailyMed - ALENDRONATE SODIUM tablet (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.