Forgetfulness has dozens of possible causes, and most of them are not dementia. Stress, poor sleep, medication side effects, vitamin deficiencies, thyroid problems, and even normal aging can all make it harder to retain or recall information. Understanding which category your forgetfulness falls into is the first step toward doing something about it.
Stress and Cortisol
Chronic stress is one of the most common and overlooked causes of forgetfulness. When you’re stressed, your body releases cortisol. In small doses, cortisol is fine. But when stress becomes chronic, cortisol levels stay elevated, and that creates problems in the hippocampus, the brain region most critical for forming and retrieving memories. The hippocampus is packed with cortisol receptors, which makes it especially vulnerable to prolonged exposure.
At low levels, cortisol actually helps with learning. But when stress pushes levels high enough to saturate those receptors, the process flips. Sustained cortisol exposure reduces the brain’s ability to strengthen connections between neurons, shrinks the generation of new brain cells in the hippocampus, and can even cause structural changes over time. This is why people going through a difficult period at work, a divorce, or financial hardship often describe feeling “foggy” or unable to remember simple things. The forgetfulness is real, it’s biological, and it typically improves when the stress is managed.
Sleep Deprivation
Sleep does far more than rest your body. During deep, slow-wave sleep, your brain consolidates memories from the day, essentially transferring them from short-term to long-term storage. At the same time, a waste-clearance system called the glymphatic system ramps up dramatically. During wakefulness, this system is mostly disengaged. During sleep, it clears roughly twice as much waste protein from the brain.
In imaging studies, just one night of sleep deprivation produced a measurable increase in amyloid-beta (a protein linked to neurodegeneration) in the hippocampus and thalamus in 19 out of 20 subjects. That clearance happens specifically during non-REM deep sleep, so even if you’re getting hours in bed, poor sleep quality can undermine the process. If you’re consistently sleeping fewer than six hours or waking frequently, that alone could explain a noticeable dip in memory.
Vitamin B12 Deficiency
Vitamin B12 plays a direct role in maintaining the protective coating around your nerve fibers, called the myelin sheath. When B12 drops too low, that coating deteriorates, and the signals between neurons slow down or misfire. The result is impaired memory, difficulty concentrating, and often a tingling or numbness in the hands and feet.
A level below 203 pg/mL is generally considered deficient, but neurological symptoms, including cognitive problems, can appear at levels between 298 and 350 pg/mL. That means your blood work could come back “normal” while your brain is already feeling the effects. In one study of patients with low B12, over half had moderate deficiency (100 to 200 pg/mL), and about 29% fell into a mildly deficient range (200 to 350 pg/mL) that would be missed by standard cutoffs. Low B12 also raises homocysteine levels, which can cause silent brain injury through oxidative stress. People most at risk include older adults, vegans, those with digestive conditions that reduce absorption, and heavy drinkers.
Thyroid Problems
An underactive thyroid (hypothyroidism) commonly causes fatigue, depressed mood, and cognitive difficulties that tend to cluster together. Of all the cognitive effects, memory is the most consistently affected domain. Attention, processing speed, and the ability to multitask also take a hit.
The mechanism involves how thyroid hormones reach the brain. The active form of thyroid hormone in the brain is T3, and most of it is produced locally by converting T4. In hypothyroidism, even when treatment brings blood markers like TSH back to normal, the brain may still not be getting enough active hormone in the hippocampus and frontal lobes. Animal studies show that standard thyroid replacement only partially restores the gene expression patterns that support memory and learning. This helps explain why some people on thyroid medication still report brain fog despite “normal” lab results.
Medications That Impair Memory
Several common drug classes can directly cause forgetfulness, and many people don’t connect their memory problems to something they take every day.
- Benzodiazepines (prescribed for anxiety and insomnia) increase the activity of a brain chemical that inhibits neural signaling. This calms anxiety but also disrupts the transfer of information from short-term to long-term memory. The effect is called anterograde amnesia, meaning you have difficulty forming new memories while under the drug’s influence. Spatial learning and memory recall are particularly affected.
- Anticholinergic drugs block a neurotransmitter involved in learning and memory. This category includes certain medications for overactive bladder, older antidepressants, some antihistamines, and certain antipsychotics. Exposure to these drugs as a class increases the odds of cognitive impairment by about 45%. In healthy volunteers, a single dose of an anticholinergic drug produced memory impairment similar to what’s seen in early Alzheimer’s disease.
If you started a new medication around the time your forgetfulness began, that connection is worth raising with whoever prescribed it. In many cases, alternatives exist that don’t carry the same cognitive burden.
Alcohol and Thiamine Deficiency
Moderate to heavy drinking affects memory through multiple pathways, but the most serious involves thiamine (vitamin B1). In people who drink heavily over long periods, poor nutrition and reduced gut absorption deplete thiamine levels, which can lead to a condition called Wernicke-Korsakoff syndrome. The early stage, Wernicke’s disease, causes confusion, low energy, balance problems, and vision changes. If caught early, these symptoms can be reversed with prompt treatment.
Left untreated, it progresses to Korsakoff’s psychosis, which involves severe, often permanent memory loss. People with Korsakoff’s lose the ability to form new memories and may unconsciously fill in gaps by inventing stories about events that never happened. They can also experience hallucinations, repetitive speech, and a loss of motivation. This progression is considered a medical emergency at the early stage because the window for reversal closes quickly.
Post-Viral Brain Fog
Since the COVID-19 pandemic, viral infections have become a widely recognized trigger for persistent forgetfulness. Long COVID brain fog affects memory, concentration, and word-finding, sometimes for months or years after the initial infection. Recent research using advanced brain imaging has identified a biological basis: widespread increases in the density of a specific receptor type in the brain, linked to both cognitive impairment and ongoing inflammation. This confirms that post-viral brain fog is a measurable, physical condition, not a psychological one.
Normal Aging vs. Something More
Some degree of forgetfulness is a normal part of aging. You might take longer to recall a name, walk into a room and forget why, or need more effort to learn something new. These lapses are frustrating but not dangerous. The key distinction is whether forgetfulness disrupts your daily functioning.
Mild cognitive impairment, or MCI, sits between normal age-related forgetfulness and dementia. The diagnostic criteria, established in 1999 and still used today, require all five of the following: memory complaints are present, daily activities remain normal, general cognitive function is intact, memory performance is below what’s expected for your age and education, and the person does not meet the threshold for dementia. MCI doesn’t always progress to dementia. In some cases, it stabilizes or even improves, especially when a treatable cause like B12 deficiency, thyroid dysfunction, or medication side effects is identified.
If you or someone close to you is concerned, screening tools used in primary care can help clarify the picture. The Montreal Cognitive Assessment (MoCA) is particularly useful because it detects MCI with about 90% sensitivity, even in people who score normally on simpler tests. It takes about 10 minutes and covers memory, attention, language, and executive function. A score below 26 out of 30 typically prompts further evaluation.
What Makes Forgetfulness Reversible
The most important thing to know is that many causes of forgetfulness are treatable. B12 supplementation can restore cognitive function when deficiency is the culprit. Adjusting or switching medications can eliminate drug-induced memory problems. Treating an underactive thyroid improves memory in many patients. Getting consistent, high-quality sleep allows the brain’s waste-clearance system to function properly. Managing chronic stress reduces cortisol exposure to the hippocampus.
The causes that tend to be irreversible, like advanced Korsakoff’s psychosis or progressive dementia, generally involve prolonged, untreated damage. That’s what makes early attention to forgetfulness worthwhile. Most of the time, the explanation is something fixable, and the sooner you identify it, the better the outcome.