Some state minimum wage adjustments are tied to the consumer price index, but otherwise, cost-of-living raises are not required by state or federal law. You just need to make sure you’re paying any applicable state and municipal minimum wages. That said, if the cost of living has increased in your area, you might want to raise wages (if you can afford it) to stay competitive in the market.
According to to Missouri Department of Labor:
2019 Missouri Minimum Wage Summary Posters
All employers subject to any provisions of the law shall post a summary of the law and regulations. The Division of Labor Standards has created a summary to post for the convenience of Missouri employers. You also may print a Spanish Minimum Wage Summary Poster. Please post the summary in a conspicuous and accessible place at the business. Required by Missouri Revised Statutes, Section 290.522.