Hello and welcome to healthcaremagic Inflammatory Acne are notorious for leaving behind pigmentation and scaring. Oral antibiotics work well for inflammatory acne and bring about rapid control. These can be doxycycline or azithromycin or minocycline. Topical treatment can be in the form of Topical Antibacterials like Benzoyl peroxide gel/Clindamycin gel for day time use and Topical Retinoids e.g Adapalene gel at night. Regarding forehead pigmentation, you should use a broad spectrum sunscreen, preferably gel based, that which offers both UV B as well as UV A protection. A skin lightening cream containing one or more of the following skin lightening ingredients e.g kojic acid, glycolic acid, arbutin, magnesium ascorbyl phosphate, liquorice etc can be used for the dark patches on forehead. I would suggest a few peeling sessions from a dermatologist, for both post acne hyperpigmentation as well as forehead pigmentation regards