A sharp drop in their stock-market valuations will have an outsize impact on cannabis companies. Deal making is already slowing, while debt is becoming scarce and more expensive.

The North American Marijuana Index, which tracks the leading cannabis stocks in the U.S. and Canada, has fallen more than 52% in the past 12 months. Valuations have come down as high prices for legal products have made it hard to shift demand out of the black market in states like California. Meanwhile, governance problems at listed companies such...