Why they're prescribed: Antihistamines are used to relieve or prevent the symptoms of allergic disorders (such as hay fever) or the common cold. Some antihistamines are also used to prevent motion sickness, nausea, vomiting and dizziness, and to treat anxiety or insomnia.
How they can cause fatigue: Antihistamines are CNS depressants. Most list tiredness and drowsiness among the potential side effects; some list dizziness and weakness.
Older people generally should not use diphenhydramine (Benadryl) — found in any sleep aid with a name ending in "PM" — because of its powerful sedative effects, which dramatically increase the risk of falls and bone fractures.
Fatigue is also a listed side effect of fexofenadine (Allegra), another popular antihistamine. Older people generally should not use this drug because their renal systems are unable to efficiently clear it, allowing the drug to build up in the body.
Alternatives: Newer-generation antihistamines such as loratadine (Claritin) and cetirizine (Zyrtec) are better tolerated by older patients and present a lower risk for fatigue.
Why they're prescribed: While antidepressants are typically used to treat depression, they're also frequently prescribed for anxiety disorders, eating disorders, obsessive compulsive disorder, chronic pain, smoking cessation and some hormone-mediated disorders, such as severe menstrual cramps.
There are many different kinds of antidepressants, including tricyclic antidepressants, monoamine oxidase inhibitors (MAOIs), selective serotonin re-uptake inhibitors (SSRIs), dopamine antagonists and lithium, among others.
Commonly prescribed antidepressants include duloxetine (Cymbalta), fluoxetine (Prozac), paroxetine (Paxil), sertraline (Zoloft), and venlafaxine (Effexor).
How they can cause fatigue: Fatigue is a common side effect of antidepressant medications, especially during early weeks of treatment. Because antidepressants may also worsen fatigue brought on by another condition, teasing out causes is often difficult.
Researchers have suggested that antidepressants may cause fatigue through their effect on the brain chemicals called neurotransmitters — chiefly serotonin and norepinephrine — and on hormones. Disruptions in the body's secretion of the stress hormone cortisol, for example, have been closely linked to fatigue symptoms.
Antidepressants can also lower levels of potassium (hypokalemia), which in turn can lead to muscle weakness.
Alternatives: Talk with your doctor or other health care provider about adjusting your antidepressant dose (side effects are often dose-related) or whether nondrug therapies might work just as well or better for you than a drug. You might also want to explore switching drugs, especially if you're older and taking one of the tricyclic antidepressants, which are considered to be potentially inappropriate drugs for older people.
Why they're prescribed: Antipsychotics are used to treat schizophrenia, bipolar disorder and other serious psychiatric conditions. Antipsychotics also are often prescribed "off label" to treat agitation and depression, among other conditions.
Commonly prescribed antipsychotics include aripiprazole (Abilify), haloperidol (Haldol), olanzapine (Zyprexa), risperidone (Risperdal) and ziprasidone (Geodon).
How they can cause fatigue: The antipsychotics are powerful CNS depressants. In studies, the side effects include fatigue, lethargy and weakness. Many of these drugs also lower dopamine levels, which can lead to drowsiness and sleepiness as well.
Alternatives: Talk with your doctor or other health care provider about the possibility of reducing dosage or switching to another antipsychotic drug. And if you've been prescribed an antipsychotic to help you sleep or for some other off-label use, you'd be well-advised to seek out more proven, safer treatments for your symptoms. (For instance, in older patients, especially, these drugs increase the risk of heart failure.)