Image source: Getty Images.

With the British pound and the euro both sinking after Britain's vote to leave the EU, stock markets are in turmoil, and a handful of bargain big pharma stocks just got a bit cheaper. 

Not long ago I noted a handful of big pharma stocks trading at ridiculously cheap prices. Let's revisit Roche (OTC:RHHBY), Novartis (NYSE:NVS) and Allergan (NYSE:AGN) to see if they're a buy following the post-Brexit sell-off.

1. Roche: Still first in class

While companies that record sales in euros and British pounds may deserve the beating they're receiving, the Swiss franc has lost less than a point today, making Roche's 5% loss in afternoon trading a bit of a head scratcher. As a result, the big pharma stock is now trading at about 16 times this year's earnings expectations.

Sure, the company's two leading drugs, Rituxan and Herceptin, comprised 28% of the company's $12.9 billion in sales last year, and they're getting old -- which means their days of patent protection are numbered. The good news is that emerging products are poised to offset their eventual losses, and the company's late-stage pipeline has it well positioned for growth in the years ahead.

The company's first-in-class anti-PD-L1 drug Tecentriq, is similar to Bristol-Myers Squibb's Opdivo except it acts on the tumor cell surface, while Opdivo acts on immune cells. Both drugs make it hard for tumors to hide from the immune system, and Opdivo's first-quarter sales this year rose a whopping 48% over the previous quarter to $704 million.

Image source: Roche.

Tecentriq earned its first approval in May for treatment of bladder cancer -- an indication Opdivo is not approved for -- and this slight differentiation from Bristol's therapy should help Tecentriq avoid competition. It has produced some exciting results in the larger lung cancer indication, and the FDA is considering an application that would expand its use to these patients, with a decision expected in October.

Roche also has a multiple sclerosis therapy, Ocrevus, on deck that is widely expected to become the first disease-slowing treatment for 10% to 15% of patients with a more aggressive form of the disease, and later the wider relapsing population. Both Tecentriq and Ocrevus have potential to reach annual sales in excess of $5 billion. This makes Roche look like a real bargain at recent prices.

2. Novartis: Another Swiss bargain

Novartis also records revenue in Swiss francs, and the currency's 1% slide against the dollar today will lower the annual dividend Americans receive. But let's not forget that Novartis has increased its distribution, in its own currency, for 19 consecutive years. With a yield of about 2.9%, this is a solid choice for both income and growth.

Novartis has a lot of irons in the fire. While most eyes are on its early-stage progress engineering immune cells to combat cancer, its commercial-stage heart product, Entresto, is what has me excited. This drug is approved for about 2.2 million U.S. patients with a form of heart failure, having proved that it reduces their chances of death from heart attack or stroke by 20% and cuts down on expensive hospitizalitions. Its price of about $4,560 per year has kept it from gaining ground, but it's about to take off.

Image source: Novartis.

A recent study published in the The Journal of the American Medical Association showed that Entresto's increased benefit over the standard of care not only reduced patients' risk of heart attack, but did so at a rate that makes it cost-effective. This makes peak annual sales estimates of $10 billion easier to swallow than the twice-daily pill.

Some stellar early-stage data from immune cell cancer therapy CTL019 bodes well for Novartis' future in oncology: The drug drove 55 of 59 patients' difficult-to-treat leukemia into complete remission. At present, the stock's price of about 15 times forward earnings, along with Entresto's potential to become the new standard of care for millions of heart failure patients, makes Novartis look like an incredible bargain.

3. Allergan: The smart acquisitor

Several years ago Allergan began growing rapidly through acquisition -- a strategy that doesn't always work out for investors. Since the end of 2013, through the trailing 12-month period, this company's top line has grown an astounding 585%. 

AGN Revenue (TTM) Chart

AGN Revenue (TTM) data by YCharts.

While it's relatively easy to boost sales when using your own shares to acquire other companies, the hard part is making those acquisitions work for investors. This is what sets Allergan apart from many of its industry peers. Over the past several years it has increased free cash flow per share at a rate that's on pace to overtake the dilutions.

Hypertension combination treatment Byvalson was recently approved, and an application is pending to expand Allergan's constipation drug Linzess (co-marketed with Ironwood Pharmaceuticals) to an additional 35 million adults, which could help the drug reach peak annual sales of $2 billion. Combined, this could put Allergan in a position to provide handsome returns over the years ahead. At about 13 times forward earnings estimates at recent prices, Allergan shares are another big pharma bargain.

While the Brexit referendum results were terribly upsetting for investors, the panic it has caused in the markets -- and the losses it has dealt these three big pharma stocks -- have created bargain opportunities for investors to take advantage of.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.