Frown lines are the result of gathering or knitting the tissue between the eyebrows into a fold. These unattractive folds and furrows are caused by the repeated action of underlying muscles associated with facial expression. Years of squinting and frowning tend to leave deep wrinkles in the skin between the eyebrows and on the bridge of the nose, across the forehead, and at the corners of the eyes. On many people, frown lines produce an angry or sad look that detracts from a pleasant facial appearance.  These lines can be improved without surgery and without scars by a simple injection of botulinum toxin type A.

Botulinum toxin is a purified toxin and, when given orally in large amounts, it blocks the nerve signals from the brain to the muscle, causing a generalized paralysis called botulism.However, by injecting very tiny amounts into a specific facial muscle, only the targeted impulse of that muscle will be blocked, causing a local relaxation and weakness. In this way, botulinum toxin acts as a muscle blockade to immobilize the underlying cause of the unwanted lines and prevent "wrinkly" expressions. 
Botulinum toxin was first used by the eye surgeon Alan Scott in 1973 to treat eye muscle disorders. Since then its use has been expanded to such conditions as entropion, dry eye , eye ticks and uncontrolled blinking (blepharospasm) , abnormal neck movements, and many others.

Botulinum toxin is injected directly into the target muscle to treat vertical lines between the eyebrows and on the bridge of the nose, the squint lines or crows feet at the corners of the eyes, the forehead horizontal lines. It may also be used for eyebrow positioning, and in facial palsy.
Once the muscle is weakened and relaxed, it cannot contract. Since there is no way to make the undesirable facial expression, the lines gradually smooth out from disuse, and new creases are prevented from forming. Other muscles like those needed to raise the eyebrows are not affected so a natural expression is maintained. Botulinum toxin may not be as effective on the smile lines around the mouth because the muscle action in this area is needed for expression and important functions like eating.

Treatment involves injection of very small amounts of botulinum toxin into the underlying muscles to immobilize them. The actual treatment is well tolerated and takes just a short time with minimal recovery. Botulinum toxin takes effect about 3 to 7 days after treatment. The improvement generally lasts about 3 to 4 months before the effect gradually fades and muscle action returns. It is anticipated that the average patient will require reinjection at various intervals. With repeated treatments, atrophy (thinning) of the muscle may occur which usually produces longer-lasting results.

Side effects are minimal and typically relate to the local injection. Soreness or mild bruising,while uncommon, may occur around the injection site. Makeup may be worn after treatment, but care should be taken to avoid pressing or massaging the area for several hours. In rare instances, patients may develop temporary weakness of the neighbouring muscles, a temporary droopy brow or eyelid, or a headache.