Topical retinoids have been used to treat acne for almost four decades and are recommended as a first line treatment option either on their own or in combination with other treatments for mild acne. They are also recommended as a first line treatment option for moderate and severe acne in combination with other treatments.
Adapalene has been available since before the millennium but historically tretinoin was the only available topical retinoid. Use of tretinoin was limited due to side effects of local irritation, which are not as problematic for 3rd generation topical retinoids like adapalene.
Topical retinoids including adapalene are especially good at treating comedones (black heads and white heads), and also have anti-inflammatory effects.
Monotherapy (used on its own)
A randomized trial of 297 patients treated with adapalene 0.1% solution or tretinoin 0.025% gel found both treatments to be clinically and statistically effective reducing inflammatory (47% and 50% respectively) and non-inflammatory acne lesions (57% and 54%) compared to baseline. 
There have been many other similar trials similarly showing adapalene is as effective as tretinoin, which has been shown by a meta-analysis combining results of five studies with 900 patients in total.
This showed that although adapalene was as effective as tretinoin it tends to work faster, and is tolerated better in terms of local side effects.
If monotherapy is not effective, combination of topical agents can be used, or topical agents in combination with oral antibiotics.
Adapalene is available as a combination gel with benzoyl peroxide. This has the advantage of being good against comedones, and it does not contain antibiotics so there are not concerns about resistance, however irritation can be a problem. This combination gel has been studied in a randomized, double blind, controlled study of 1670 patients. Patients were given the combination product, adapalene alone, benzoyl peroxide alone or placebo. They found that the combination product was significantly more effective at reducing inflammatory and non-inflammatory acne lesions compared to monotherapy.
Adapalene has also shown to work well in combination with antibiotics and it is actually recommended that oral antibiotics for acne should be combined with topical benzoyl peroxide or a topical retinoid. One study of 467 patients showed that a combination or oral doxycycline with topical adapalene was more effective at treating inflammatory and non-inflammatory acne lesions compared to oral doxycycline alone.
Oral isotretinoin (roaccutane) is currently the gold standard treatment for severe acne. A study of 266 patients comparing adapalene/benzoyl peroxide combination gel and oral doxycycline against oral isotretinoin concluded that although isotretinoin was more effective, adapalene/benzoyl peroxide combination gel with oral doxycycline had a better side effect and safety profile than isotretinoin and should be considered for patients intolerant, unable or unwilling to take oral isotretinoin.
Adapalene has also been shown to be effective in maintaining acne free skin after previous successful acne treatment.
A randomized controlled trial of 253 participants who had received successful treatment in the previous 12 weeks with either adapalene and doxycycline or doxycycline and placebo gel, were then randomized to receive either adapalene maintenance therapy or placebo for the next 16 weeks. The adapalene group resulted in a significantly larger proportion of patients with successful maintenance compared to the placebo group (75% vs 54%).
In a similar trial it has also been shown to be an effective maintenance treatment after successful lymecycline.
Combination treatment of adapalene with benzoyl peroxide has been shown to be successful maintenance therapy after treatment with isotretinoin.