A bisphosphonate for bone protection
Alendronic acid
A bisphosphonate that strengthens bone and lowers fracture risk in osteoporosis, taken with strict instructions to protect the food pipe.
What is Alendronic acid?
Alendronic acid is a bisphosphonate used to treat and prevent osteoporosis, the condition where bones become thin and fragile. It works by slowing the cells that break down bone, helping to keep bones stronger and reduce the chance of fractures.
Education and reference only. This is a plain-language guide to Alendronic acid — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Alendronic acid is one of the most widely used medicines for osteoporosis — the condition where bones become thinner and more fragile and break more easily. It is taken to strengthen bone and reduce the risk of fractures, particularly of the spine and hip, and is used in people at increased fracture risk, including after the menopause and in those taking long-term steroids. It is usually a once-weekly tablet taken with very specific instructions, and it works quietly over months and years to protect the skeleton.
How it works
Bone is constantly being broken down and rebuilt by two types of cell. In osteoporosis, the cells that break bone down (osteoclasts) outpace the rebuilders, so bone is gradually lost. Alendronic acid sticks to the bone surface and damps down the osteoclasts, slowing the breakdown of bone. With breakdown reduced, bone density is preserved or improved over time, which is how the medicine lowers the chance of fractures.
Company & origin
Originated / developed by: Merck & Co. (originally Istituto Gentili).
Alendronate was first synthesised in the 1970s and identified as a potent bone-resorption inhibitor by Istituto Gentili in Italy. Merck & Co. licensed it in 1988, reformulated it as the sodium salt, and gained FDA approval for the osteoporosis drug Fosamax in 1995.
What it treats
Conditions Alendronic acid is used for
Practical use
How to take Alendronic acid
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it first thing in the morning on an empty stomach, before any food, drink or other medicines.
- Swallow it whole with a full glass of plain water only, not tea, coffee, juice or mineral water.
- Stay sitting or standing upright for at least half an hour afterwards, and do not lie down.
- Wait at least half an hour before eating, drinking anything else or taking other tablets.
- Tell your dentist you take it before any tooth extraction or major dental work, and report any new thigh, hip or groin pain.
Weighing it up
Advantages & disadvantages of Alendronic acid
Advantages
- Reduces the risk of spine and hip fractures in osteoporosis.
- Often taken just once a week, making the strict routine easier to manage.
- Long-established treatment with extensive evidence.
- Low-cost and widely available.
Disadvantages
- Has a strict dosing routine that must be followed to work safely and effectively.
- Can irritate the gullet and cause heartburn, difficulty swallowing or chest pain if taken incorrectly.
- Rarely linked to jaw bone problems and unusual thigh-bone fractures with long-term use.
- Not suitable for people who cannot sit or stand upright, or who have certain swallowing problems.
- Calcium and vitamin D levels usually need to be adequate for it to work properly.
Practical use
Good to know
The way it is taken matters as much as the medicine itself, because the tablet can irritate the food pipe (oesophagus) if it lingers there. Take it first thing in the morning on an empty stomach, swallow it whole with a full glass of plain tap water (not other drinks), and then stay fully upright — sitting or standing, not lying down — and do not eat, drink anything else or take other medicines for a while afterwards. Only after that waiting period should you have breakfast. This routine helps the tablet reach the stomach and protects the food pipe. Calcium and vitamin D levels usually need to be adequate, so a supplement is often taken too (at a different time of day). A dental check is sensible before or during treatment, and any thigh or groin pain should be reported.
Who should not take it / use with caution
- People who cannot sit or stand upright for the required time after taking it, or who have problems with the food pipe (such as narrowing or delayed emptying).
- People with low blood calcium (this is corrected first), or with significantly reduced kidney function.
- Used with care in people with active dental or jaw problems, who may be advised to have dental treatment beforehand.
Monitoring
- Bone density (usually by DEXA scan) over time
- Calcium and vitamin D status, and kidney function
- Dental health, and any thigh/groin pain or swallowing problems
Side effects
- Indigestion, heartburn, difficulty or pain on swallowing, or tummy upset — often linked to not following the upright/empty-stomach routine.
- Muscle, bone or joint aches, especially when starting.
- Rarely, irritation or ulcers of the food pipe; very rarely, osteonecrosis of the jaw (a problem with jaw-bone healing) and unusual "atypical" thigh-bone fractures — report new or unusual thigh, hip or groin pain.
Key interactions
- Calcium and vitamin D supplements, antacids, and iron must be separated from it — take alendronic acid first and these later — because they stop it being absorbed.
- Food, milk, coffee, juice and mineral water all reduce its absorption, which is why only plain water is used.
- Anti-inflammatory painkillers (NSAIDs) can add to the risk of stomach and food-pipe irritation.
Available as: Tablets (commonly a once-weekly tablet, sometimes a daily one). A liquid form exists for people who cannot swallow tablets, taken with the same upright, empty-stomach precautions.
Answers
Alendronic acid: frequently asked questions
Why do I have to stay upright after taking alendronic acid?
The tablet can irritate or even ulcerate the food pipe if it sticks there instead of reaching the stomach. Taking it with a full glass of plain water and staying upright, without eating or drinking anything else for a while, helps it pass down quickly and protects the food pipe. This routine is the single most important part of taking it safely.
Why does it have to be plain water and an empty stomach?
Alendronic acid is poorly absorbed, and food, milk, juice, coffee and even mineral water dramatically reduce how much gets into the body. Taking it first thing on an empty stomach with plain tap water gives it the best chance to work, which is why other drinks and breakfast must wait.
What is osteonecrosis of the jaw and should I worry?
It is a rare problem where an area of jaw bone fails to heal, usually after dental surgery. It is uncommon with the tablets used for osteoporosis, but it is why a dental check before or during treatment is sensible and why you should keep up good mouth care and tell your dentist you take a bisphosphonate.
Should I report thigh or groin pain?
Yes. New, unusual or persistent pain in the thigh, hip or groin can rarely be an early sign of an "atypical" thigh-bone fracture linked to long-term bisphosphonate use. Report it so it can be checked, rather than assuming it is ordinary aching.
What is the difference between alendronic acid and Fosamax?
They are the same medicine — alendronic acid is the generic (active-ingredient) name and Fosamax is a brand name. Generic alendronic acid contains the identical active ingredient.
The wider class
About Bisphosphonates
Alendronic acid belongs to the bisphosphonates class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
Browse by body system
Authoritative sources
- BNF: Alendronic acid.
- electronic Medicines Compendium (SmPC): Alendronic acid (Fosamax).
- NICE CKS: Alendronic acid.
- NICE: Osteoporosis — bisphosphonates.
Building a medicines information resource?
We create evidence-led, dose-free drug and formulary references for teams.