The Securities and Exchange Commission is cracking down on companies that use the Black-Scholes formula to value complicated warrants in an effort to get them to switch to more sophisticated methods.

The problem comes when companies rely on Black-Scholes to value warrants that can be exercised early or have provisions like a down round feature that protects investors in case the company goes out to raise additional funds, says Tony Alfonso, president of BDO Valuation Advisors.

As a closed-form valuation model, Black-Scholes takes into account only the stock price, the strike price and the instrument's term, and not additional complexities, Alfonso says. "Where the SEC has come out is cautioning folks that you cannot use Black-Scholes for that," he says. "You have to use an open-form model, either a lattice model, binomial, or a Monte Carlo simulation."