Pimples seem to be the commonest skin disease amongst many, especially within the young folks.
It most often affects the face, back, chest, and shoulders. This is because there are many sebaceous glands in these areas of skin.
Dandruff is a condition of the scalp that causes flakes of skin to appear. It is often accompanied by itching.
Dandruff can trigger acne on the forehead as it is basically a skin infection. It is caused by fungi that can also irritate the skin on your forehead and cause pimples.
Your hair gets oily when sebaceous glands in your scalp produce excess sebum, which travels down the hair shaft. If you have an oily scalp, the oil can run down and result in clogged pores and acne on your forehead.
Medications like birth control pills tend to trigger forehead acne in certain people. Consult/question your doctor about the medication he has prescribed if this happens.
Exfoliating is an important step in every beauty regimen. As they always say, too much of everything is bad. Over-exfoliating can use pimples, breakouts, puffiness and shiny forehead.
Emotional stress causes breakouts. It can disrupt the hormonal balance and also the regular oil production of the skin, causing pimples.
If you know you are going to go through trying times, you can be very careful with skincare.