Lithium is one of the oldest and most effective psychiatric medications, primarily used to stabilize mood in bipolar disorder. When you take it, the drug works on multiple signaling systems in your brain simultaneously, but the effects aren’t instant. Acute mania typically takes 6 to 10 days to respond, and depressive symptoms can take 6 to 8 weeks to improve. What happens in between, and over the long term, involves real changes to brain chemistry, body weight, kidney function, and thyroid health.
How Lithium Changes Brain Chemistry
Lithium is a simple ion, chemically similar to sodium and magnesium. Once it enters your bloodstream and crosses into the brain, it affects mood through at least two major pathways. The most studied involves an enzyme called GSK-3, which plays a central role in regulating mood, energy, and cellular health. Lithium blocks this enzyme directly by competing with magnesium, which GSK-3 needs to function. It also blocks it indirectly by boosting the activity of another protein (Akt) that naturally keeps GSK-3 in check.
The second major pathway involves dopamine, the neurotransmitter tied to motivation, pleasure, and reward. Dopamine signaling is thought to be dysregulated in bipolar disorder, particularly during manic episodes. Lithium interferes with a specific dopamine receptor signaling chain, effectively dampening the exaggerated dopamine activity associated with mania while leaving baseline function relatively intact. This dual action on both the enzyme and the dopamine system helps explain why lithium works for both the “highs” and “lows” of bipolar disorder, though it’s notably stronger against mania.
What the First Weeks Feel Like
Because lithium takes 6 to 10 days to begin working against mania, it’s almost always started alongside faster-acting medications like antipsychotics or sedatives. Those drugs handle the acute crisis while lithium builds up in your system. For depression, the wait is longer: 6 to 8 weeks before you may notice a meaningful shift in mood. During this ramp-up period, your prescriber will draw blood to check your lithium level and adjust your dose accordingly.
The target blood level for long-term maintenance is 0.60 to 0.80 mmol/L. If you’re tolerating the medication well but not getting enough benefit, the level may be pushed to 0.80 to 1.00 mmol/L. If side effects are bothersome but your mood is stable, it can be lowered to 0.40 to 0.60 mmol/L. Going above 1.00 mmol/L doesn’t appear to add further benefit for maintenance and increases the risk of toxicity.
Common Side Effects
Lithium’s side effects are common enough that most people on the drug will experience at least one. In a survey of 237 long-term lithium patients, about two-thirds reported increased thirst, and roughly half experienced a noticeable hand tremor. The tremor is typically fine and most visible when holding something like a cup or writing. Thirst and frequent urination go together: lithium affects the kidneys’ ability to concentrate urine, so your body pushes more water through.
Weight gain is another frequent concern. A prospective study found that the average gain was about 4 kilograms (roughly 9 pounds). This isn’t universal, but it’s common enough that it becomes a real factor in whether people stay on the medication long term. The weight tends to accumulate gradually rather than all at once.
Effects on Your Thyroid
Lithium slows thyroid hormone production, and over time this can tip into clinical hypothyroidism. In an Italian study of real-world psychiatric patients, about 8.7% developed hypothyroidism significant enough to require thyroid hormone replacement. A much larger group, around 37%, showed temporary thyroid changes that resolved on their own without stopping lithium or adding another medication.
This is why thyroid monitoring is a standard part of lithium treatment. Thyroid-stimulating hormone (TSH) levels are typically checked every three to six months during the first year, then every six to twelve months after that. If hypothyroidism develops, it’s usually manageable with a daily thyroid supplement, and most people don’t need to stop lithium.
Long-Term Kidney Effects
The kidney question is the one that concerns most people on lithium for years or decades. Lithium is cleared entirely by the kidneys, and over time it can reduce their filtering capacity. The risk depends heavily on your age when you start and how long you take it.
For people who begin lithium between ages 18 and 54, the 10-year risk of developing severe kidney disease (stage 4 or higher) is essentially zero, ranging from 0% to 0.7%. The risk climbs with age: those starting between 55 and 74 face a lifetime risk of 13.9% to 18.6%, with the 65 to 74 age group most vulnerable. Interestingly, people starting at 75 or older had a lower lifetime risk of just 5.4%, likely because they have fewer remaining years of exposure. Twenty or more years on lithium was associated with a six-fold increase in severe kidney disease compared to one to five years of use.
These numbers mean kidney monitoring through regular blood tests is non-negotiable for anyone on lithium, but they also mean the risk for younger patients over a typical treatment course is quite low.
Lithium’s Protective Effects on the Brain
One of lithium’s most striking properties is that it appears to protect brain tissue. Bipolar disorder is associated with progressive loss of gray matter, particularly in the hippocampus, a region critical for memory and emotional regulation. Neuroimaging studies consistently show that lithium-treated patients have hippocampal volumes comparable to healthy people without bipolar disorder, while untreated patients show significantly smaller volumes.
This neuroprotective effect holds up even in patients who continue to have mood episodes while on lithium. In one study, patients who experienced breakthrough bipolar episodes despite taking lithium still had hippocampal volumes indistinguishable from healthy controls and significantly larger than patients not on lithium. The protection appears to come from the medication itself, not simply from having fewer episodes.
Toxicity: A Narrow Safety Margin
Lithium has one of the narrowest therapeutic windows of any commonly prescribed medication. The difference between an effective dose and a toxic one is small, which is why regular blood monitoring matters so much.
Mild toxicity (blood levels of 1.5 to 2.5 mmol/L) causes nausea, vomiting, increased tremor, lethargy, and fatigue. Moderate toxicity (2.5 to 3.5 mmol/L) brings confusion, agitation, delirium, rapid heart rate, and muscle stiffness. Severe toxicity (above 3.5 mmol/L) can cause seizures, coma, dangerously low blood pressure, and overheating. Since the therapeutic range tops out around 1.0 mmol/L, toxicity begins not far above that ceiling.
Dehydration is the most common trigger. Anything that reduces your body’s water volume, whether illness with vomiting or diarrhea, heavy sweating, or simply not drinking enough, concentrates lithium in your blood. Certain medications are also risky. Common painkillers like ibuprofen and naproxen reduce the kidney’s ability to clear lithium by restricting blood flow to the filtering structures. Blood pressure medications including ACE inhibitors and some diuretics have similar effects. If you take lithium, any new medication, even an over-the-counter one, is worth checking against your lithium level.
Pregnancy Considerations
Lithium use during the first trimester has long been associated with a rare heart defect called Ebstein’s anomaly. A large study published in the New England Journal of Medicine put the actual numbers in perspective: cardiac malformations occurred in 2.41% of lithium-exposed infants compared to 1.15% of unexposed infants. The specific type of heart defect most closely linked to lithium, a right ventricular outflow tract obstruction, occurred in 0.60% of exposed infants versus 0.18% of unexposed infants. The risk is real but far smaller than earlier case reports suggested, and it needs to be weighed against the significant risks of untreated bipolar disorder during pregnancy.