They may charge high prices hence exploiting customers.
Due to lack of competition, they can produce substandard goods.
Causes unequal distribution of income.
Too much monopoly power may influence the government to adopt unfavorable policies.
Denies customers a variety of goods and services.
Monopolists may overlook some markets which are less profitable hence denying customers in those markets access to goods and services.
Monopolists may cause artificial shortages so as to manipulate prices.