People with allergies to lavender may experience skin irritation after contact, and should avoid lavender in all forms.

Mild rash can develop after applying lavender oil. Reports describe increased sun sensitivity and changes in skin pigmentation after applying products containing lavender oil. Nausea, vomiting, loss of appetite, constipation, headache, chills, confusion and drowsiness are sometimes reported after inhaling lavender or absorbing it through the skin, or after large doses of lavender or perillyl alcohol (derived from lavender) are taken by mouth. The essential oil of lavender may be poisonous if taken by mouth.

Drowsiness can occur after lavender aromatherapy. More severe drowsiness or sedation may occur when lavender is used with other sedating agents. Use caution if driving or operating heavy machinery. In theory, lavender used by mouth may increase the risk of bleeding. Individuals with bleeding disorders or taking drugs that may increase bleeding should use caution. Dosing adjustments may be necessary.

Some cancer patients have experienced low blood cell counts (neutropenia) after taking high doses of perillyl alcohol by mouth.

Lavender is not recommended during pregnancy or breastfeeding.

Lavender has been taken by mouth as a tea prepared from 1 to 2 teaspoons (5 to 10 grams) of leaves steeped in 1 cup (250 milliliters) of boiling water for 15 minutes. As a tincture, 60 drops (1:5 in 50% alcohol) per day has been used.

Lavender oil has been used in aromatherapy (inhaled) and massage therapy (applied on the skin). A naturopathic eardrop called NHED, which includes lavender, has been used at a dose of 5 drops three times a day with or without an antibiotic and topical anesthetic.

To reduce perineal discomfort after childbirth, 6 drops of lavender oil have been added to a bath. Another technique is to add 1/4 to 1/2 cup of dried lavender flowers to hot bath water.

Early cancer studies report doses of 800 to 1,200 milligrams per square meter of body surface, taken by mouth, four times daily in a 50:50 perillyl alcohol (a derivative of lavender):soybean oil preparation.

There is not enough scientific evidence to safely recommend lavender for children.

Animal studies suggest that lavender used as aromatherapy or by mouth may increase the amount of drowsiness caused by some drugs. Examples include benzodiazepines such as lorazepam (Ativan®) or diazepam (Valium®), barbiturates such as phenobarbital, narcotics such as codeine, some antidepressants, and alcohol. Drowsiness caused by some seizure medicines may also be increased. Caution is advised while driving or operating machinery.

In theory, lavender may add to the effects of cholesterol-lowering drugs.

Lavender may have additive effects when used with prescription antidepressant medications, such as the tricyclic antidepressant imipramine.

Lavender may increase the risk of bleeding when taken with drugs that increase the risk of bleeding. Some examples include aspirin, anticoagulants ("blood thinners") such as warfarin (Coumadin®) or heparin, anti-platelet drugs such as clopidogrel (Plavix®), and non-steroidal anti-inflammatory drugs such as ibuprofen (Motrin®, Advil®) or naproxen (Naprosyn®, Aleve®).

Lavender used as aromatherapy or by mouth may increase the amount of drowsiness caused by some herbs or supplements, such as valerian. Caution is advised while driving or operating machinery.

In theory, lavender may add to the cholesterol-lowering effects of some herbs or supplements such as fish oil, garlic, guggul, and niacin.

Lavender may increase the risk of bleeding when taken with herbs and supplements that are believed to increase the risk of bleeding. Multiple cases of bleeding have been reported with the use of Ginkgo biloba, and fewer cases with garlic and saw palmetto. Numerous other agents may theoretically increase the risk of bleeding, although this has not been proven in most cases.