PEPTO-BISMOL, the US antacid drug, was first marketed in New York at the turn of the 20th century. 

Its active ingredient is bismuth subsalicylate, technically a “salt” of bismuth. Bismuth salts have been known for their use in treating stomach upsets and bowel problems for centuries.

The medicine is effective for mild stomach ulcers, diarrhoea, and indigestion. One of the most surprising things about it is that it’s not known how bismuth subsalicylate actually works. 

Although a simple and small molecule, there remain many different theories as to how the compound makes sufferers feel better. There doesn't have to be a single right answer: many are likely to play a role in reducing symptoms.

Bismuth itself is a metal. It is an ancient metal which has been known throughout history, unlike some metals which weren’t recognised as separate substances until more recently. 

Bismuth is striking for its resemblance to lead, although it lacks lead’s toxicity and is therefore an ideal substitute in many applications.

Under careful conditions, the metal bismuth can be crystallised to form perfect cube shapes. But when the crystallisation happens quickly, the crystals are “hoppered.” 

The term comes originally from the cone-shaped “hoppers” used to funnel grain in mills. The outer edges of cubes form but the crystallisation process is too fast for them to be filled in, producing a beautiful ridged pattern like a staircase.

The cuboid and hoppered crystals are made all the more beautiful by a very thin layer of oxide on the external surface. 

Just like rust, the outer surface of the pure metal reacts with oxygen to produce a very thin layer on the outside. The crust on the outside prevents the penetration of oxygen into the rest of the metal. 

This crust is so thin that it interacts with light like the thin surface of a bubble, producing vibrant oily colours.

Crystals are what happens when atoms or molecules find their natural equilibrium and stack in an ordered pattern with each other. The purer the material, the more perfect the crystal, because every molecule in the material has a shape and size in common. 

This uniformity produces near perfect stacking, which stacking recapitulates the symmetry of the material at the atomic scale all the way up to the scale on which it is easily visible to us. 

Because bismuth is an elemental metal, the crystals that it makes in its pure form are simple and clearly faceted.

When elements react to form larger and more complicated compounds, the ways in which they crystallise change. Depending on the form of the molecule, the crystal can give important insights into the shape of the compound itself. 

Developments in the use of this method were responsible for working out the helical structure of DNA, along with many other biological molecules. 

It is still a vital tool in the work of chemists and biochemists who want to understand the structures of complex biological molecules.

In crystallography a very short wave, much smaller than the size of light, is shot through the crystalline material. This is done with X-rays, or with electrons or neutrons. 

The way that the waves pass through the material is strongly affected by the crystal’s repetitive structure, producing spots of intensity defined by the tiny structures. 

An analogue of these patterns is seen in laser displays where red and green lights are spread into arrays of spots. 

In order to get really high-intensity and high-quality beams of X-rays and subatomic particle beams, across the globe, large accelerator facilities have been built in which scientists can apply to use a part of the beam over a period of time. 

One facility in Britain is the Diamond Light Source in Harwell, Oxfordshire. The facility is 86 per cent funded by the British government, and 14 per cent funded by the Wellcome Trust, a philanthropic trust founded on the wealth generated from the pharmaceutical owner Henry Wellcome.

As well as generating the beam, one of the major issues is the ability to crystallise large enough high-quality crystals. There must be relatively few defects inside the structure. 

Although multiple small crystals can also be tested in the form of a powder, additional disorder inside tiny crystals can complicate working out their internal structure.

To return to Pepto-Bismol, bismuth subsalicylate has small molecules that naturally form small needle-shaped crystals. These are much bigger than an atom in length but the narrow dimension is much smaller than the wavelength of visible light. It is these crystals suspended in water that give Pepto-Bismol its pink colour.

Until last month, the structure of these crystals was not known. However, new work by researchers in Sweden took a different approach to studying the material. 

Instead of attempting to grow a large enough crystal to do traditional crystallography, the researchers used an electron microscope to look at the materials, and to measure the spacings of the molecules inside the crystals.

What they found was that the small molecules of the saltarrange themselves in ultra-thin layers, which are stacked on top of each other throughout the crystal. 

In each of the layers most of the molecules have more or less the same direction, but some of the layers are oriented in the opposite direction. 

It is this randomness inside the material which has made the molecular arrangement of the chemical compound so hard to understand until now. 

It took the careful measurement of the crystals on a plane-by-plane level of focus to see how the molecules build the tiny crystals in the medicine. 

There are still many questions about the drug, as to how exactly it functions as a drug in the body, but this step may well help understand that.

The drug works, and has worked since long before the crystal structure was understood, but the careful unpicking of the chemistry of everyday life is still a work in progress. 

As in many drugs and materials of all kinds, we understand most about the effects we can sense or feel, less about the mechanism that causes those, and even less about the fundamentals of the material itself.