Avoid in individuals with a known allergy or hypersensitivity to members of the Lamiaceae (mint) family or to any component of thyme, or to rosemary (Rosmarinus officinalis).
Cross-reactions to birch pollen, celery, oregano, and to other species in the Lamiaceae/Labiatae (mint) family may occur. Symptoms of allergy may include nausea, emesis (vomiting), pruritus (severe itching), angioedema (swelling under the skin), dysphagia (difficulty swallowing), dysphonia (altered voice), hypotension (low blood pressure), and progressive respiratory difficulty. Occupational asthma has been reported.
Although not well studied in humans, thyme flowers and leaves appear to be safe in culinary and in limited medicinal use. Caution is warranted with the use of thyme oil, which should not be taken by mouth and should be diluted when applied on the skin due to potentially toxic effects.
Side effects of thyme taken by mouth may include headache, dizziness, hypotension (low blood pressure), bradycardia (slowed heart rate), heartburn, nausea, vomiting, diarrhea, gastrointestinal irritation, muscle weakness, and exacerbated inflammation associated with urinary tract infections. Use cautiously in patients with gastrointestinal irritation or peptic ulcer disease.
Taking thyme oil by mouth may also cause seizure, coma, cardiac arrest, or respiratory arrest. High doses of thyme or thyme oil may elicit tachypnea (rapid breathing). Inflammation of the eye and nasal mucosa has also been reported with exposure to thyme dust.
Topical application of Listerine® antiseptic solution to a chronic parenchyma of the toe has caused inflammation of the skin. Avoid topical preparations in areas of skin breakdown or injury, or in atopic patients. As an ingredient in toothpaste, cases of inflamed lips and tongue have been attributed to thyme oil.
Although not well studied in humans, Thymus serpyllum, a related species to Thymus vulgaris, has been shown to exert effects on the thyroid. Use cautiously in patients with thyroid disorders. Estradiol and progesterone receptor-binding activity has also been demonstrated.
Thyme is not recommended in pregnant or breastfeeding women due to a lack of available scientific evidence. Thyme may act as an emmenagogue (promotes menstruation) and abortifacient (promotes abortion).
Theoretically, thyme may decrease levels of thyroid hormone. Patients taking thyroid replacement therapy or anti-thyroid agents should use cautiously. Monitoring may be necessary.
Although not well studied in humans, thyme may interact with agents with estrogen or progesterone receptor activity. Examples of agents that may be affected include hormone replacement therapies and birth control pills.
Topical (applied on the skin) thymol may increase the absorption of 5-fluorouracil. Caution is advised in chemotherapy patients, as 5-fluorouracil is often used in cancer chemotherapy. Consult with a qualified healthcare professional, including a pharmacist, to check for interactions.
Although not well studied in humans, thyme may interact with herbs with estrogen or progesterone receptor activity. Caution is advised when combining thyme with other herbs and supplements with proposed hormonal effects, such as black cohosh.