Which medication is known for potentially causing anxiety and insomnia?

The medication known for potentially causing anxiety and insomnia is Synthroid. Synthroid contains levothyroxine, which is a synthetic form of the thyroid hormone thyroxine (T4). This medication is used to treat hypothyroidism, a condition where the thyroid does not produce enough hormones. When the dosage is too high or not appropriately managed, it can lead to an excess of thyroid hormones in the body, which increases metabolism and can contribute to symptoms such as anxiety, insomnia, irritability, and restlessness. These effects are most pronounced if the patient's thyroid levels are over-corrected.

In contrast, the other medications listed do not typically cause anxiety and insomnia as side effects. Sinemet is used for Parkinson’s disease and mainly affects motor function. Singulair is an asthma medication that tends to help with respiratory issues rather than mood or sleep. Tylenol is a pain reliever and fever reducer that does not have a significant impact on anxiety or sleep patterns. Thus, Synthroid is the medication most associated with the potential for increased anxiety and insomnia due to its impact on thyroid hormone levels.