Can I mix 2 different -2% hydroquinone creams to get 4% strength?

There are common precipitors for melasma that include hormones (estrogen and progesterone), birth control pills, stress, thyroid problems, sun exposure, and inflammation. In my practice, I use a combination of topicals: 
Melarase AM and Melarase PMMelapadsMelaclenzReplenish Retinoid CreamOnce started, we combine this regimen with fractional laser, pulsed dye laser, Covalent chemical peels, and LED Yag. 
Best, 
Dr. Karamanoukian
Realself100 Surgeon

Unfortunately, mixing two 2% hydroquinone creams does not equal a 4% strength. Make sure you are under the supervision of a provider. Hydroquinone is a great product, but you don't want to be on it long term. Best, Dr. Emer

I recommend that you use the 2% HQ product with a non HQ product like Melarase AM.BestH Karamanoukian MD

Thanks for asking your question.  Combining two different hydroquinone of the same strength won't double the strength.  It would remain the same.  You have to be careful with hydroquinone use long term because of potential complications to include acute and chronic issues. Acute complications include irritant or allergic contact dermatitis and postinflammatory hyper- and hypopigmentation.  Chronic issues include ochronosis, nail discoloration, conjunctival melanosis, and corneal degeneration.  Ochronosis is the most common chronic complication related to long-term use of hydroquinone, and is comprised of asymptomatic hyperpigmentation, erythema, papules, papulonodules, and grey-blue colloid milia on sun-exposed areas of the skin.  There are some carcinogenicity concerns as well.  Short-term hydroquinone use for a few months is fine up to 4% and often can get better results mixing with Kojic acid, retinoids, and steroids.  Some countries have banned hydroquinone because of the concerns of people over using them.I do prescribe these at times to "jump start" improvement of hyperpigmenation, but just for a few months.  However, will then stop it.  Then start SkinCeuticals "Advanced Pigment Corrector because it has a tyrosinase inhibitor and is safe for daily use pretty much indefinitely.  It can improve both discoloration and even the skin tone evenness.  
We have had tremendous success using this product and like it because it has been clinically proven and tested on all ethnicities to improve the appearance of stubborn skin discoloration and uneven skin tone as it boosts the appearance of radiance. Skin discoloration is often caused by accumulated sun exposure, hormonal shifts, and the natural aging process, Advanced Pigment Corrector is a multi-action comprehensive technology working to exfoliate skin to reduce the appearance of discoloration.  Fades the appearance of discoloration, promotes even skin tone, improves visible skin luminosity for renewed radiance, ideal for nearly all skin types, except very sensitive, paraben-free, and non-comedogenic.  In summary the best thing is it doesn't carry the potentially very serious risks hydroquinone can and it can be used daily indefinitely.Lastly a "physical" type sunblock with zinc or titanium vs a chemical sunblock is key to keeping the pigment from flaring up again when exposed to outdoor sunlight.I hope this was helpful.  Best regards.

No, that will still be a 2% concentration in different bases. You are better off with a COMPOUNDED solution based upon your type of melasma and skin type. Most dermatologist compound creams from 2 % all the way to 8% for melasma. We make it a point to give you a concentration depending on your type of melasma, your skin sensitivities, and your previous success and or failures. Every melasma patient should be treated differently as there are many combinations to treat this condition. Melasma can not be completely cured, however effective treatment are possible. This is because your skin is extremely sensitive to UV and even the smallest amount can stimulate your pigment cells to produce colour. The mainstay of treatment is strict UV protection- hats, sunglasses and SPF every 4 hours.

I prefer to use a combination of creams and laser for melasma. Hydroquinone can be used (5-8%), along with vitamin A creams. I usually start my patients on laser (low dose Q switch or Picosure) a few weeks after they commence on creams. Another laser I use is the Clear and Brilliant in the Permea setting combined with creams.

In some cases I combine glycolic AHA peels, and in other cases I use a tablet called Tranexamic acid to help. Melasma treatments should be tailored to each patient.

All the best, Dr Davin Lim. Laser, surgical and aesthetic dermatologist. Brisbane, Australia