No serious negative side effects (“adverse events”) have been reported with lemon balm supplementation to date, and no side effects have been reported when it has been taken for less than 8 weeks.[1]
Allergic: Some hypersensitivity reactions, such as contact dermatitis, have been reported.[2][1]
Ophthalmological: There are some anecdotal reports of an increase in intraocular pressure.[2][1]
Cardiovascular: Although lemon balm is traditionally used to treat heart palpitations,[3] one randomized controlled trial reported that some participants experienced palpitations as an adverse event during the study.[2][1]
Neurological: In addition to lack of alertness, headaches, EEG changes, and sleep disturbances have been reported.[2][4][1]
Hormonal: There is a theoretical concern that lemon balm could have an effect on thyroid hormones, which is supported by in vitro evidence, animal studies, and one case report of lemon balm being used as a part of a successful combination treatment in a patient with treatment-resistant Graves’ disease.[5][6][7][8][9] For this reason, people with thyroid conditions might consider having their thyroid hormone levels checked when taking substantial doses of lemon balm.