From the FDA Drug Label
Each capsule for oral administration contains: Lithium Carbonate USP . . . . . . . . 150 mg, 300 mg or 600 mg The empirical formula for Lithium Citrate is C6H5Li3O7; molecular weight 209.92. Lithium carbonate USP is a white, light, alkaline powder with molecular formula Li2CO3 and molecular weight 73.89.
The FDA drug label does not answer the question.
From the Research
You should not switch from lithium tablets to lithium carbonate capsules without consulting your healthcare provider first, as different formulations may have different release rates, bioavailability, and dosing requirements, which can affect the medication's efficacy and safety. Lithium has a narrow therapeutic window, meaning the difference between an effective dose and a toxic dose is small, and blood lithium levels need to be carefully monitored 1. Changing formulations without medical supervision could lead to either insufficient treatment or dangerous side effects from toxicity.
Some lithium products are extended-release while others are immediate-release, which affects how quickly the medication enters your bloodstream. For example, a study found that two extended-release lithium carbonate products were not bioequivalent when given in single doses to healthy volunteers, with one product producing a 40% lower maximum plasma lithium concentration and the other producing a 25% lower maximum plasma lithium concentration compared to an immediate-release product 2.
Additionally, lithium treatment can be associated with increased risk of reduced urinary concentrating ability, hypothyroidism, hyperparathyroidism, and weight gain, as reported in a systematic review and meta-analysis 3. Therefore, it is crucial to discuss any concerns about side effects, cost, or availability issues with your doctor, who can safely adjust your prescription if appropriate. Your doctor has prescribed a specific formulation and dosage based on your individual needs and medical history, and any changes should be made under their supervision to ensure your safety and the effectiveness of the treatment.