Antipsychotics can cause a wide range of side effects, from weight gain and drowsiness to movement problems and hormonal changes. The specific risks depend on which medication you’re taking, how long you’ve been on it, and your individual biology. Older antipsychotics (first-generation) tend to cause more movement-related problems, while newer ones (second-generation) are more likely to trigger metabolic changes like weight gain and blood sugar spikes. Both types carry some degree of risk across these categories.
Weight Gain and Metabolic Changes
Metabolic side effects are among the most common and concerning, especially with second-generation antipsychotics. These drugs interfere with the brain’s appetite signals by blocking receptors that normally help regulate hunger and energy balance. The combination of effects on serotonin, dopamine, and histamine receptors shifts your body toward storing more energy, increasing appetite, and disrupting how insulin works.
The amount of weight gain varies dramatically by medication. In one major trial, olanzapine and clozapine caused roughly 12 kilograms (about 26 pounds) of weight gain over a year. Quetiapine and risperidone were more modest at 2 to 3 kilograms. Aripiprazole and ziprasidone caused relatively little or no weight gain. Olanzapine in particular has been shown to increase calorie intake by about 345 calories per day, enough to gain over a kilogram per week in the short term.
Beyond weight, these medications raise the risk of type 2 diabetes (reported in 3% to 28% of people taking them) and abnormal cholesterol levels (15% to 50%). Importantly, about 25% of diabetes cases linked to antipsychotics occur in people who aren’t obese, meaning the drugs can disrupt blood sugar regulation independently of weight gain. People on these medications are roughly 2.8 times more likely to develop high cholesterol and 2.3 times more likely to have elevated triglycerides compared to untreated individuals.
Movement Problems
Antipsychotics work partly by blocking dopamine pathways, and dopamine is also essential for smooth, coordinated movement. This creates a category of side effects called extrapyramidal symptoms, which affect a significant number of people. A global review estimates that about 20% of people on antipsychotics develop drug-induced parkinsonism, 11% experience akathisia, and 7% develop tardive dyskinesia. First-generation antipsychotics cause these problems far more often than newer ones.
These movement side effects take different forms:
- Parkinsonism: Tremors, stiffness, and slowed movement that resemble Parkinson’s disease. This is the most common movement side effect.
- Akathisia: An intense inner restlessness and inability to sit still. People describe it as one of the most distressing side effects, sometimes severe enough to make them want to stop treatment entirely.
- Dystonia: Sudden, involuntary muscle contractions that can twist the neck, lock the jaw, or distort facial expressions. These tend to appear early in treatment.
- Tardive dyskinesia: Repetitive, involuntary movements, usually of the face, tongue, and jaw. Unlike the others, this develops after months or years of use and can be permanent.
Tardive Dyskinesia Risk Over Time
Tardive dyskinesia deserves special attention because it can persist even after stopping the medication. The annual incidence is about 6.5% per year with first-generation antipsychotics and 2.6% per year with second-generation drugs. That means with each passing year on medication, the cumulative risk grows. Newer antipsychotics cut the risk roughly in half compared to older ones, but they don’t eliminate it. Once tardive dyskinesia develops, it may or may not improve after discontinuation, which is why catching early signs matters.
Hormonal Effects
Many antipsychotics raise levels of prolactin, a hormone normally involved in milk production. When prolactin stays elevated, it causes a cascade of problems. In well-conducted studies, about 45% of women on conventional antipsychotics experienced irregular or absent periods, and 19% developed galactorrhea (breast milk production unrelated to pregnancy). Both men and women can experience sexual dysfunction, breast tissue growth, and reduced fertility. These effects are more common with first-generation antipsychotics and risperidone, while medications like aripiprazole tend to have less impact on prolactin.
Heart Rhythm Changes
All antipsychotics can affect the electrical activity of the heart to some degree, prolonging something called the QT interval. When this interval stretches too far, it raises the risk of dangerous heart rhythm disturbances. Among the most studied medications, thioridazine and ziprasidone cause the greatest prolongation, while haloperidol and olanzapine cause the least. Both haloperidol and thioridazine have appeared on a World Health Organization list of the 20 drugs most commonly linked to a specific dangerous arrhythmia called torsades de pointes. If you have existing heart conditions or take other medications that affect heart rhythm, this risk becomes more relevant.
Sedation and Cognitive Effects
Drowsiness is one of the most immediate and noticeable side effects, particularly with medications like clozapine, olanzapine, and quetiapine. This happens because these drugs block histamine receptors, the same system targeted by sleep-inducing antihistamines. For some people, sedation is mild and fades over the first few weeks. For others, it remains a persistent problem that affects work, driving, and daily functioning. Cognitive dulling, difficulty concentrating, and a general feeling of mental sluggishness are related complaints that people on antipsychotics frequently report.
Neuroleptic Malignant Syndrome
Neuroleptic malignant syndrome is rare but potentially fatal. It involves severe muscle rigidity, high fever, mental status changes, and unstable blood pressure and heart rate. It can occur with any antipsychotic at any point during treatment, though it’s most common early on or after a dose increase. Early mortality rates exceeded 30%, but with better recognition and faster treatment, the current mortality rate is below 10%. If you develop an unexplained high fever with muscle stiffness while taking an antipsychotic, this is a medical emergency.
Clozapine and Blood Cell Monitoring
Clozapine is uniquely effective for treatment-resistant schizophrenia, but it carries a specific risk that no other antipsychotic shares: agranulocytosis, a dangerous drop in white blood cells that leaves the body unable to fight infections. This affects roughly 1% of people taking clozapine, with the highest risk during the first month. Because of this, the FDA requires weekly blood draws for the first six months, then every two weeks for the next six months, and monthly after that. This mandatory monitoring schedule is the price of access to what is often the most effective option for people who haven’t responded to other treatments.
Increased Risk in Older Adults With Dementia
The FDA issued a black box warning, its most serious safety alert, after a review of 17 trials found that elderly patients with dementia who took atypical antipsychotics had 1.6 to 1.7 times the risk of death compared to those on placebo. Over a typical 10-week trial, 4.5% of drug-treated patients died compared to 2.6% on placebo. Most deaths were cardiovascular (heart failure, sudden death) or infectious (pneumonia). These medications are not approved for treating dementia-related psychosis, though they are still sometimes used when other approaches fail.
What Monitoring Looks Like
Because metabolic side effects can develop quickly and silently, guidelines recommend a structured monitoring schedule. Your weight should ideally be checked weekly for the first four to six weeks, then every two to four weeks through the first three months, then at six months and annually. Blood pressure, fasting blood sugar, and cholesterol levels follow a similar pattern: checked at baseline, at 4, 8, and 12 weeks, and then periodically after that. Blood sugar is best measured with a fasting test in the early weeks and with a longer-term marker (hemoglobin A1c) over time. If you switch antipsychotics, the monitoring clock resets. These checks exist because metabolic changes can start within the first few weeks, well before you’d notice symptoms on your own.