At first glance, the rise of Commonwealth Bank of Australia (ASX: CBA) shares over the past 18 months has been nothing short of unprecedented. In many ways, that's true.  

For one, no ASX bank (or possibly any bank) has been valued at more than 30 times earnings before. Yet CBA commands a price-to-earnings (P/E) ratio of 31.66 at present. 

For another, no ASX share had ever been valued at more than $300 billion before Commonwealth Bank crossed that Rubicon earlier this month.

Experts have been calling CBA out as overvalued for as long as anyone can probably remember now. Yet that didn't stop the bank rising from $112 a share at the start of 2024 to the all-time high of $183.19 that we saw cracked this week.

This apparent defiance of investing gravity has earned short-selling CBA shares the nickname of the 'widowmaker' trade in some circles. Many ASX investors hold this ASX 200 bank stock, with some having owned shares since the bank's stock market debut back in the early 1990s. As such, many investors probably want some insights into what might happen next. 

While no one can decisively answer that question, such is the nature of the markets, I thought it might be a good idea to compare CBA shares to another Australian investing stalwart – healthcare giant CSL Ltd (ASX: CSL). 

A quick look at CBA and CSL will probably yield little recognition. After all, both companies are as different as chalk and cheese on most levels. CSL is an international healthcare leader, specialising in vaccinations and blood plasma medicines. CBA, in contrast, is our nation's largest bank and financial institution, with little overseas presence.

But it wasn't too long ago that CSL shares were experiencing something very similar to what CBA has been going through.

Does CSL give us a roadmap for CBA shares?

CSL shares have been stuck in the mud for years now. At just under $20 a share today, the company is asking the same price as it was exactly six years ago.

Yet, many investors might not know that, in the years prior to 2019, CSL could do no wrong. Its shares went from $64 each in August 2014 to quickly rise to $100 by late 2015, then $130 in 2017, $200 in 2018, and $250 in 2019. The company topped out at over $330 a share in 2020, which happens to be CSL's most recent all-time high. 

Between 2015 and 2020, CSL just kept going up, with seemingly nothing standing in the way of the next milestone. Fund managers were sceptical of the ever-rising P/E ratio, but afraid to let go of the company, lest they underperform and look silly. Shorting CSL was the widowmaker trade of the day.  

Check it out below:

As you can see, it wasn't to last. With the onset of the COVID-19 pandemic, CSL finally fell back to earth in 2020. The company has still been growing its earnings and revenues at a fairly steady rate ever since. But its share price has stagnated. CSL once traded on an earnings multiple of close to 50. Today, it is at 27.6. 

I think the similarities between the CSL of 2019 and the CBA of 2025 are striking similar. I also believe that CSL's fate is one that CBA is destined to share. When will the company finally come back to reality? Well, that's the $183 question. But I wouldn't be betting that the bank can defy logic forever.