The most common cause of weak and brittle hair is improper care and careless handling of the strands. Weak hair is also caused by poor diet, hormonal problems, dandruff and other scalp diseases, chronic stress, alcohol and cigarette abuse or even taking certain medications. In most cases, a change in eating habits or less stress is enough, but sometimes the problem will require stronger support in the form of aesthetic medicine treatments for the scalp, such as needle mesotherapy.