Stimulant side effects are well characterised, mostly mild, and mostly settle in the first two weeks of any given dose [1, 2]. This article covers the common side effects of methylphenidate and lisdexamfetamine, why each one happens, the timeline along which they usually settle, and the practical adjustments that help.
How stimulants produce the side-effect profile they do
ADHD stimulants raise dopamine and noradrenaline activity in specific brain circuits, but those neurotransmitters also signal in other parts of the body. Noradrenaline drives the sympathetic nervous system: heart, blood pressure, gastrointestinal motility and salivary glands. Dopamine has roles in appetite, mood and arousal. The side-effect profile of stimulants is the predictable downstream consequence of doing useful work in the brain while also nudging these other systems.
This is why the side effects of stimulants tend to be:
- Reduced appetite (dopamine and noradrenaline both suppress hunger signalling)
- Mild rises in heart rate and blood pressure (sympathetic activation)
- Difficulty getting to sleep (noradrenergic arousal late in the day)
- Dry mouth (reduced parasympathetic salivary stimulation)
- Headache (cardiovascular shift and vasoconstriction in some patients)
- Mood flattening or rebound irritability as the dose wears off (the brain catching up after the medication peak)
Knowing the cause makes the effects more predictable and the adjustments more sensible.
The common side effects, in order of how often they show up
Reduced appetite
The most consistent side effect across both methylphenidate and lisdexamfetamine [1, 2]. Appetite is most suppressed when the medication is at its peak, which for stimulants is the middle of the day. Lunchtime is the meal most affected.
Practical adjustments:
- Eat a solid breakfast before the medication takes effect
- Use the evening meal, when the medication is wearing off, to recover calorie and protein intake
- Snacking on calorie-dense, low-volume options (nuts, cheese, smoothies) in the late afternoon helps where weight loss is becoming a concern
- For children, the school day picture matters; close communication with the prescriber about weight and growth is part of ADHD medication and titration with NeuroFX
Sleep difficulties
Difficulty getting to sleep is the most common sleep effect, often in the first two weeks. The cause is residual noradrenergic activity later in the day. The detail is covered in the dedicated article on medication and sleep in this library.
Practical adjustments:
- Take the dose as early in the morning as practical
- Avoid late-afternoon top-ups where the prescriber has not specifically prescribed them
- Maintain a regular evening wind-down: dim light, limited screens, consistent bedtime
- Caffeine after midday is amplified by stimulant medication and is worth limiting
Headache
Often present in the first one to two weeks; usually settles. The mechanism includes the cardiovascular shift and, for some patients, mild dehydration on the day's medication. Practical adjustments include hydration (one extra litre of water spread across the day for most adults), paracetamol where appropriate and not contraindicated, and ensuring the morning dose is taken with food.
If headache is severe, persistent past two weeks, or new in pattern (visual symptoms, focal neurology, vomiting), it is not a stimulant settling-in effect and warrants prompt review.
Dry mouth
Common, usually mild, and easily managed with hydration and sugar-free gum or lozenges. Persistent dry mouth contributes to dental problems if left unaddressed; regular dental review remains important.
Heart rate and blood pressure changes
A small rise in resting heart rate (typically a few beats per minute) and blood pressure (typically a few millimetres of mercury) is common and usually well tolerated in healthy patients [3, 4]. The dedicated article on the cardiovascular evidence in this library covers the longer-term picture.
Practical adjustments:
- Baseline measurements before starting medication
- Repeat measurements through titration and at every review
- A home blood pressure monitor is inexpensive and useful for patterns; not essential
- Persistent or significant changes are a conversation, not a reason to stop without prescriber input
Mood changes and rebound
Some patients experience a flat mood, irritability, or a noticeable dip as the medication wears off in late afternoon (often described as "rebound"). This is more common with immediate-release formulations and less common with long-acting modified-release options.
Practical adjustments:
- A short conversation about formulation choice. A different release profile can soften rebound
- A small immediate-release top-up at the right time, where the prescriber agrees
- Recognising that the wear-off period is not the same as the medication "not working"
Irritability and emotional lability
Distinct from rebound, this can occur during the active medication window in some patients, particularly in children and adolescents. It often settles within two to four weeks of a steady dose. If it does not, it is a reason for the prescriber to consider a different stimulant or a non-stimulant.
Reduced growth in children
Slowed growth in weight, and sometimes in height, is reported in children on long-term stimulants and is mentioned in both the BNFc and the BNF [1, 2]. The reduction is usually modest and is reversible: growth typically returns to expected trajectory after the medication is stopped. Routine monitoring of weight and height at each review is part of NICE NG87 [4]. Some patients use a brief medication-free period (often the summer holidays) to allow growth catch-up; this is a clinical decision made with the prescriber, not a default.
Less common but worth knowing
The BNF lists less frequent effects including tics, sustained low mood, hallucinations, significant cardiovascular changes and growth concerns [1, 2]. These are uncommon but are reasons to contact the prescriber rather than waiting.
A timeline for what to expect
Most early effects settle within ten to fourteen days of a given dose. Where a dose increase brings back some effects, those usually settle again within a similar period. By six weeks at a steady working dose, the side-effect profile is usually stable: the picture you have at week six is broadly the picture you will have at week twelve.
A calm guide to the first six months on stimulants covers the longer stretch.
When a side effect is a stop-the-medication issue
Most side effects are reasons to adjust dose, timing or formulation, not reasons to stop. A small set of effects warrant prompt review or temporary discontinuation pending review [1, 2]:
- New chest pain or shortness of breath
- Fainting
- Persistent very high heart rate or blood pressure
- New onset suicidal thinking or significant mood deterioration
- New seizures
- New tics that are severe or distressing
- Hallucinations
- Signs of allergic reaction
If any of these appear, contact your prescriber the same day, or use 999 or A&E for acute presentations.
What this means in practice
- Most stimulant side effects are mild and settle within the first two weeks of any given dose.
- Reduced appetite, mild sleep disturbance and a small rise in heart rate are the most common, and all have practical adjustments.
- The side-effect profile at six weeks on a stable dose is usually the profile at twelve weeks.
- Side effects that do not settle are a reason to talk to the prescriber, not a reason to stop on your own.
- A small set of effects (chest pain, fainting, severe mood change, hallucinations, severe tics) are reasons for prompt review.
When to speak to a professional
Speak to your GP, your prescriber or NHS 111 if a new symptom appears that worries you, if your blood pressure or heart rate feels noticeably different, or if changes to mood, sleep or appetite are not settling. Seek urgent help via 999 or A&E for chest pain, shortness of breath, fainting or any acute mental health crisis. NeuroFX offers private adult ADHD assessment and paediatric ADHD assessment for children aged 6 and upwards with structured titration and review.
Sources
- British National Formulary. Methylphenidate hydrochloride. https://bnf.nice.org.uk/drugs/methylphenidate-hydrochloride/
- British National Formulary. Lisdexamfetamine mesilate. https://bnf.nice.org.uk/drugs/lisdexamfetamine-mesilate/
- British National Formulary. Dexamfetamine sulfate. https://bnf.nice.org.uk/drugs/dexamfetamine-sulfate/
- NICE. Attention deficit hyperactivity disorder: diagnosis and management. NG87. National Institute for Health and Care Excellence. https://www.nice.org.uk/guidance/ng87
- Cortese S, Adamo N, Del Giovane C, et al. Comparative efficacy and tolerability of medications for attention-deficit/hyperactivity disorder in children, adolescents, and adults: a systematic review and network meta-analysis. Lancet Psychiatry. 2018;5(9):727-738.
