Alpha-2 agonists
Medicines that turn down the body’s adrenaline-style “fight or flight” signals. Born as blood pressure tablets, now better known for ADHD, tics and sleep. Calming, but clingy: they don’t like being dumped suddenly.
At a glance
- For
- ADHD, tics, blood pressure, menopausal hot flushes, withdrawal symptoms, sedation in hospital
- Works
- ADHD: a modest benefit, smaller than stimulants (in a key guanfacine trial, about 6 in 10 children responded versus 4 in 10 on a dummy treatment). Hot flushes: in small studies, clonidine cut flushes by 38% versus 24% on a dummy treatment
- Cost
- Clonidine: on the PBS. Guanfacine: PBS for ADHD in children and teens, with an authority
- Kicks in
- Drowsiness within hours; ADHD benefit builds over 2–4 weeks
- Taken for
- Months to years, reviewed regularly
- Watch for
- Sleepiness, dizziness on standing, low blood pressure, slow pulse, dry mouth
They calm the nervous system, not the breakup. Stop them suddenly and blood pressure can bounce back with interest.
Brand names
- Clonidine: Catapres, plus generics. Clonidine guide
- Guanfacine: Intuniv (slow-release tablets). Guanfacine guide
- Methyldopa: generics
- Moxonidine: Physiotens
- Dexmedetomidine: Precedex (hospital only)
How it works
Alpha-2 receptors act as a brake on the release of noradrenaline, the body’s own adrenaline-type messenger. Switching them on slows the heart, lowers blood pressure and has a calming, sedating effect. Guanfacine is more targeted to the type found in the front of the brain, which handles attention and impulse control.
Used for
- ADHD, alone or added to a stimulant (guanfacine; clonidine off-label in Australia)
- Trouble getting to sleep with ADHD, usually in children (clonidine, off-label; evidence limited)
- Tics and Tourette syndrome, especially with ADHD (off-label)
- High blood pressure (clonidine, moxonidine; methyldopa in pregnancy)
- Menopausal hot flushes (clonidine)
- Easing withdrawal from opioids or alcohol, under supervision (clonidine, off-label)
- Sedation in intensive care and during procedures (dexmedetomidine)
Compared
Clonidine and guanfacine do most of the work outside hospital. The others are niche.
- The all-rounderClonidine (Catapres)
- How taken
- Tablets, usually 2 or 3 times a day
- Approved in Australia
- High blood pressure (other uses are off-label)
- Used for
- ADHD and ADHD-related sleep trouble, tics, hot flushes, withdrawal, blood pressure
- Sleepiness
- Very common
- Stopping
- Reduce slowly over more than a week. Rebound high blood pressure is more likely at high doses
- Notable
- A slow-release form is approved for ADHD in the US. Even one tablet can be dangerous to a small child
- ADHD specialistGuanfacine (Intuniv)
- How taken
- Slow-release tablet once a day; swallow whole
- Approved in Australia
- ADHD in children and teens aged 6–17
- Used for
- ADHD, alone or with a stimulant; tics with ADHD (off-label); some adults (off-label)
- Sleepiness
- Common, especially early on
- Stopping
- Reduce slowly; blood pressure can rebound
- Notable
- Not a stimulant and not addictive. High-fat meals and grapefruit raise levels
- PregnancyMethyldopa
- How taken
- Tablets 3 to 4 times a day
- Approved in Australia
- High blood pressure
- Used for
- High blood pressure in pregnancy (one of the first choices)
- Sleepiness
- Drowsiness can occur, especially early on
- Stopping
- Your doctor will plan any change
- Notable
- Avoided in people with a history of depression, anxiety or postnatal depression
- Blood pressureMoxonidine (Physiotens)
- How taken
- Tablet once or twice a day
- Approved in Australia
- High blood pressure
- Used for
- Blood pressure, usually as an add-on
- Sleepiness
- Possible
- Stopping
- Reduce slowly
- Notable
- Not on the PBS
- Hospital onlyDexmedetomidine (Precedex)
- How taken
- Drip into a vein
- Approved in Australia
- Sedation in intensive care and during surgery or procedures
- Used for
- Calm, rousable sedation
- Sleepiness
- That’s the point
- Stopping
- Managed by the hospital team
- Notable
- Can slow the heart and lower blood pressure
Also in the family: lofexidine (opioid withdrawal, approved in the US and UK) and brimonidine eye drops (glaucoma).
Not the same as alpha blockers. Prazosin and tamsulosin block a different receptor (alpha-1) to relax blood vessels or the prostate. Different family, different job.
Side effects
Sleepiness, tiredness, dizziness on standing and a dry mouth are the usual ones. They often ease after the first few weeks.
Full side-effect list (from the product information)
How often, from the clonidine (Catapres) product information. Guanfacine is similar, with sleepiness, headache, tummy pain and tiredness the most common. Bold = serious.
Very common more than 1 in 10
- Dizziness
- Drowsiness or sedation
- Light-headedness on standing (low blood pressure)
- Dry mouth
Common 1 in 10 to 1 in 100
- Low mood
- Sleep problems
- Headache
- Constipation
- Nausea or vomiting
- Pain in the salivary glands
- Erection problems
- Tiredness
Uncommon 1 in 100 to 1 in 1,000
- Delusions or seeing things that aren’t there
- Nightmares
- Pins and needles
- Slow heart rate
- Cold, pale fingers (Raynaud’s)
- Itch, rash or hives
- Feeling generally unwell
Rare 1 in 1,000 to 1 in 10,000
- Breast swelling in men
- Heart block (an electrical problem causing a very slow pulse)
- Dry eyes or a dry nose
- Severe bowel slow-down (colonic pseudo-obstruction)
- Hair loss
- Raised blood sugar
Frequency not known reported, rate unclear
- Confusion
- Lower sex drive
- Blurred near vision
- Slow or irregular heart rhythm
Get help urgently if
- Fainting, a very slow pulse or feeling very faint
- Extreme sleepiness or slowed breathing
- Chest pain or a racing, irregular heartbeat
- A child swallows any tablets not meant for them: call 000 or the Poisons Information Centre on 13 11 26 (Australia)
Good to know
- Don’t stop suddenly. Blood pressure can rebound, with agitation, headache and a racing heart. Doses are reduced slowly.
- Alcohol, sleeping tablets and other sedating medicines add to the drowsiness. Take care driving until you know how it affects you.
- Blood pressure, pulse and, in children, height and weight are checked regularly.
- Store locked away from children.
Common questions
Is clonidine or guanfacine better for ADHD?
Both help less than stimulants. Guanfacine is the one approved for ADHD in Australia and is taken once a day; clonidine is used off-label, often when sleep is a problem. The choice depends on symptoms, timing, side effects and other conditions.
Can I stop clonidine or guanfacine suddenly?
No. Blood pressure can rebound and cause agitation, headache and a racing heart. Your doctor will reduce the dose slowly.
Are alpha-2 agonists the same as alpha blockers?
No. Alpha blockers such as prazosin and tamsulosin block alpha-1 receptors to relax blood vessels or the prostate. Alpha-2 agonists turn down the brain’s adrenaline-type signals.
Is clonidine a sleeping tablet?
It isn’t approved as one. It’s sometimes used off-label to help children with ADHD fall asleep, but the evidence is limited.
Information only. Not a recommendation or endorsement of any product. Whether a medicine suits you depends on your health and other medicines, so talk to your doctor or pharmacist. In an emergency, call 000 (Australia) or your local emergency number. From a mobile, 112 works in most countries.
More information
- Catapres (clonidine) product information
- Australian Prescriber: Guanfacine for ADHD
- NPS RADAR: Guanfacine for ADHD (PBS criteria and trial results)
- American Academy of Neurology guideline on tics (summary)
- Australasian Menopause Society: Non-hormonal treatments for menopausal symptoms
- SOMANZ: Managing high blood pressure in pregnancy
- Healthdirect: Physiotens (moxonidine)
- Precedex (dexmedetomidine) product information
- Healthdirect: Medicines information
maladywise.com/medicines/alpha-2-agonists/