A standard DAO Botox treatment uses 2 to 4 units per side, for a total of 4 to 8 units. The injection targets the depressor anguli oris, the small muscle responsible for pulling the corners of your mouth downward. By relaxing it, Botox allows the corners to lift slightly, softening marionette lines and reducing that “resting frown” appearance.
What the DAO Muscle Does
The depressor anguli oris runs from the jawbone up to the modiolus, a small hub of muscle fibers at each corner of the mouth. Its job is to pull the lip corners downward and widen the mouth. It works in balance with other muscles around the lips, so when it contracts strongly or dominates over time, the mouth takes on a downturned look even when your face is relaxed.
Botox works here the same way it works anywhere else: it blocks the chemical signal (acetylcholine) that tells the muscle to contract. With the DAO relaxed, the opposing muscles that lift the mouth corners meet less resistance, creating a subtle upward shift. The result isn’t a dramatic change. It’s a softening of the downward pull that can make your resting expression look more neutral or pleasant.
Why the Dose Stays Low
The 2 to 4 units per side range is intentionally conservative. The DAO sits close to another muscle called the depressor labii inferioris (DLI), which controls the lower lip. The two muscles overlap along their inner borders, and the DLI sits deeper and more toward the center of the chin. If Botox drifts into the DLI, it can cause the lower lip to roll inward or create an uneven smile.
A case report published in dermatologic literature described exactly this scenario: toxin that diffused medially into the DLI on one side paralyzed it, leaving the opposite lower lip pulling downward unopposed. The result was visible asymmetry. Keeping the total dose small and placing injections precisely reduces this risk considerably.
Where Injections Are Placed
Injectors typically use two common sites along each DAO muscle. One is just to the outside of the base of the marionette line, right along the jawline. The other is in the middle of the marionette line itself. Both sites stay lateral enough to avoid the DLI.
To locate the DAO, your injector may ask you to pull the corners of your mouth downward so the muscle becomes visible under the skin. Another technique involves feeling for the modiolus at the mouth corner and mapping a fan-shaped zone extending roughly 45 degrees to the outside and 30 degrees to the inside of a center line drawn down from that point. Injections go within that zone. Gentle outward massage immediately after injection helps push the toxin away from the DLI and keep it where it belongs.
Microdosing as an Alternative Approach
Some practitioners use a microtoxin technique in the lower face, injecting very dilute Botox into the skin’s surface layer rather than deep into the muscle. In published microtoxin protocols, the DAO area receives around 1 to 2 units per injection point, with tiny blebs placed just under the skin. Because the injection stays shallow, the risk of affecting deeper muscles like the DLI drops significantly.
Microtoxin is a different treatment philosophy than standard intramuscular Botox. It targets skin tightness, pore size, and surface texture across a broader area rather than paralyzing one specific muscle. If your main concern is muscle-driven downturn at the mouth corners, standard intramuscular DAO Botox at 2 to 4 units per side is the more direct approach. If you want overall skin quality improvement in the lower face with some mild DAO relaxation, microtoxin may be worth discussing with your provider.
When to Expect Results
You won’t see changes right away. Botox generally takes 2 to 5 days to begin weakening the muscle, with the full effect building over 5 to 6 weeks. In clinical data, the average onset was about 7 days. The results last roughly 2 to 3 months before the muscle gradually regains its full strength. Because the DAO dose is small, some people find the effect fades on the shorter end of that range, closer to 8 weeks.
Botox vs. Fillers for a Downturned Mouth
DAO Botox and dermal fillers address the same concern through completely different mechanisms, and the right choice depends on what’s actually causing the downturn. If the problem is an overactive DAO muscle pulling the corners down, Botox relaxes that pull. If the problem is volume loss in the cheeks or along the jawline allowing skin to sag, fillers physically support the tissue and lift the mouth corners from underneath.
Fillers tend to produce more visible results for severe marionette lines or significant downturning, and the effect is immediate. Botox produces a subtler lift that takes days to appear. For many people, the best outcome comes from combining both: Botox to quiet the muscle and filler to restore the lost volume. Your provider can help determine whether one or both treatments make sense based on what’s happening in your lower face at rest versus when you’re actively frowning.