A provider calls the pharmacy to say the cream did not work. We ask four questions: how much did you use, how thick was the layer, did you occlude, and how long did it sit. In the overwhelming majority of those calls, the answer to at least one of them explains everything. A 20/10/10 triple-anesthetic compound is not a magic trick that overrides physics. It still has to cross the stratum corneum, and that takes product, contact, and time.
What the cream is actually doing while it sits
Topical anesthetics work by diffusing through the outermost layer of skin and reaching the free nerve endings in the epidermis and upper dermis. The stratum corneum is the rate-limiting step — a layer of dead, keratin-packed cells that exists specifically to keep things out. Diffusion across it depends on the concentration gradient, how hydrated that layer is, and how long the gradient is maintained.
That is why a BLT compound uses three agents rather than one. Benzocaine at 20% moves quickly and starts the surface effect; lidocaine at 10% builds the working block; tetracaine at 10% is slower in and considerably longer out. The three curves stack. But every one of them still needs the same thing: enough drug sitting on hydrated skin, long enough to move.
Why occlusion changes the math
Plastic film occlusion approximately doubles the rate of transdermal absorption. Two things happen under the film: the stratum corneum hydrates, which opens the diffusion path, and the preparation stops evaporating and drying on the surface where it does nothing at all.
That doubling is a tool, not a freebie. Faster absorption means more drug in circulation over the same period, so occlusion should be used deliberately — for deep procedures that need rapid onset — with the total dose reduced accordingly. Never occlude broken or inflamed skin, and document occlusion in the procedure note the same way you document dose.
Dwell time by procedure
MediCaine's house protocol is straightforward, and the split is by procedure depth rather than by body part:
- Superficial procedures — 15 to 20 minutes. Botox and neuromodulator injections, IV access, superficial biopsies. The target is shallow and the needle is small.
- Deeper procedures — 20 to 25 minutes. Dermal fillers, microneedling, laser and IPL. More depth, more energy, more time needed for the block to reach the tissue that will actually be treated.
- No occlusion on a deep procedure — add 5 to 10 minutes. If you are not using film, you are running at roughly half the absorption rate, and the clock has to reflect that.
Site matters on top of procedure. Thin, vascular skin — eyelids, lips, the inner arm — responds faster and clears faster. Palms, soles, and the back are slower on both ends. Our procedure-specific pages carry the working detail: microneedling, laser, lip filler, Botox, and tattoo work.
The dose ceiling that governs all of it
For a healthy adult, the maximum single-use dose is approximately 30 g of standard 20/10/10 BLT on intact skin without occlusion. Large body-surface treatments — full back or thigh laser hair removal, for example — require divided dosing or a step-down tier rather than simply spreading more product over more skin. The compound is contraindicated on broken or inflamed skin, mucous membranes unless specifically indicated, the eyes, and large body surface areas under occlusion, and in patients with known ester-class (benzocaine, tetracaine) or amide-class (lidocaine) allergies.
This is the part where "leave it on longer" stops being a performance question and becomes a safety one. Extending contact time over a large area increases systemic absorption without meaningfully improving the block. If the block is not adequate at protocol time, the fix is usually dose, film thickness, or tier — not the clock.
Five mistakes that waste a good compound
- Too little product. A translucent smear is not a dose. The layer should be opaque and visible; you should not be able to read skin texture through it.
- Rubbing it fully in and walking away. Working a thin film into the skin and leaving nothing on the surface removes the reservoir that maintains the gradient for the next twenty minutes.
- Skipping occlusion on a deep procedure and not extending the time. Either occlude, or add the 5 to 10 minutes. Doing neither is the single most common cause of a disappointing result.
- Applying to wet skin. Cleanse the area, then let it dry. Alcohol still evaporating off the surface dilutes the film exactly where contact matters most.
- Using the wrong tier for the procedure. A tier built for superficial work will underperform on a chemical peel or an excision no matter how long it sits. See Pro vs Elite vs Ultra or the full application guide.
A protocol your whole team can run the same way
The reason dwell time drifts in a busy practice is that it lives in one person's head. Written into the room protocol — cleanse, dry, apply an opaque layer, occlude if deep, set a timer, clean before treating — it stops being a judgment call made by whoever is rooming the patient that afternoon.
If your procedure mix does not map cleanly onto the ranges above, call the pharmacy. Working out the right tier and timing for a specific case mix is a five-minute conversation, and it is the kind of question our pharmacist would rather answer before an order than after a disappointing appointment.
This article is general information for licensed providers and is not a substitute for the ordering provider's clinical judgment or the directions supplied with your preparation. Always follow your own protocol, applicable state regulations, and the labeling on the product you received.