As a cure-all? Absolutely not. As a cure-a-few-things? It worked great.

The early history of cod liver oil as a treatment is a bit mysterious—there have been attempts to trace the use of fish oils in general back as far as Classical antiquity through Hippocrates and Pliny, though no cod species seems to be specifically mentioned in these works.^(1) Another protomedical use that's often brought up is as a traditional recourse of Northern Europeans against the cold, aches, and "old pains."^(2) I haven't actually seen any mention of which Northern Europeans used cod liver oil in this way, or how that use is documented; regardless, its history as an actual (clinical) medical treatment is much more recent.

The earliest mention of cod liver oil by a modern practicing physician seems to have been that by the eighteenth-century doctor Thomas Percival, who recorded it as a treatment (for rheumatism) used by another doctor, Robert Darbey of the Manchester Infirmary:

A woman who laboured under the most excruciating rheumatism, and was an outpatient of this infirmary, being advised to rub her joints with the oil,was induced to take it at the same time internally. A few weeks restored her to the use of her limbs, and she was cured. However, little attention was paid to this case, as it was supposed that the alteration of the weather, and the medicine she had before taken, had caused the cure. About a twelvemonth afterwards, her complaints returned with double violence, and the same remedy restored her to health again. Encouraged by this second recovery, Dr. Kay (1766), one of the physicians to the infirmary, prescribed it for other patients,in similar cases ; and it answered his most sanguine expectations. Since then,it has been used by the other physicians with the greatest success.^(3)

While cod-liver oil remained quite popular in Manchester—the infirmary going through "near a hogshead annually"—its use remained relatively geographically restricted to the British Isles and Manchester more specifically. The remedy only became widely discussed in Continental circles in the 1820s. An 1822 article by the German physician Cristoph Wilhelm Hufeland focused on cod liver oil as a treatment for rheumatism and gout; in 1824, another German, D. Schütte, published an article on the substance's antirachitic properties; and by 1833, a third German doctor by the name of Henkel had reported on cod liver oil as a treatment for tuberculosis.^(4)

Before moving on, we might stop to consider these claims; it turns out that they're largely well-substantiated. Given the anti-inflammatory properties of omega-3 fatty acids, cod liver oil was and is an effective treatment for gout and rheumatism; it also contains large amounts of Vitamin D (if not quite so much as other fish liver oils^(5)) to make good the deficiencies that cause rickets. The use of cod liver oil as a cure for tuberculosis is a bit more unclear, since TB (unlike the previously-mentioned afflictions) is a bacterial disease. It may have just helped to relieve the symptoms (by reducing inflammation in the lungs), or to facilitate the body with the intake of other nutrients—a well-nourished body can resist most diseases, including TB, better than a malnourished one.^(6) This is all to say that cod liver oil does have proven abilities as a medicinal substance. So why, we might ask, is it so rare today?

Cod liver oil enjoyed an increasingly large celebrity as a treatment for various afflictions (but largely rheumatism, gout, rickets, and tuberculosis) through the mid-nineteenth century. Various acclaimed physicians touted its role as a curative for some ailment or another—Armand Trousseau popularized it for rickets, John Hughes Bennett for TB (and, if less forcefully, for rickets as well). But while cod liver oil was still being marketed to consumers into the twentieth century, medical opinion had begun to turn against it. This was because of any malignant side effects of cod liver oil treatments (though Bennett had noticed a few as early as 1841), but because of debates over what components of cod liver oil were effective and whether they could be more cheaply procured—or synthesized—elsewhere.

One hypothesis, which had its roots in the 1870 experiments of the Viennese pathologist G. Wegener, posited that phosphorus was the component responsible for cod liver oil's potency as a medical treatment. While Wegener himself was skeptical of this conclusion (since cod liver oil was the solvent he used to get the phosphorus), further tests by Max Kassowitz further suggested that cod oil's main medical use was simply as a cheap and common solvent for phosphorus. The international reputation of Viennese medicine helped cement this conclusion in the mind of the Euro-American medical community, despite further experiments that suggested that cod liver oil was somehow more potent than alternative oils.^(7)

The early twentieth century saw a resurgence in the prestige of cod liver oil with the discovery of vitamin A, which was taken to be the effective component in rickets treatment. This view persisted until 1922, when Elmer McCollum showed that cod liver oil retained its antirachitic properties even after its vitamin A had been destroyed. In the later 1920s, further work on cod liver oil compared it with UV treatment as a remedy for rickets. These experiments led to today's scientific consensus that cod liver oil's efficacy as an antirachitic substance stems from its high levels of Vitamin D.^(8)

Ihde also notes a cultural bias that might have mitigated against scientific recognition of the benefits of cod liver oil: its rather unsavory provenance. Late nineteenth-century German pharmacists preferred to synthesize drugs, looking on biologically-occurring remedies as Dreckapotheke ("dirt-medicine," a term denoting any remedy that seems filthy or disgusting) and remnants of folk medicine. Furthermore, commercial cod liver oil was often adulterated or even substituted with other, less effective oils.^(9)

But while clinicians might have considered cod liver oil to be an unsavory means to an end (i.e., phosphorus), it still remained a popular curative well into the twentieth century—at least in the US. Pharmaceutical surveys (conducted on different samples, so don't read more heavily into this data!) reveal that cod liver oil appeared in 16.4/10,000 prescriptions in 1885; 54.7/10,000 in 1895; 46.4/10,000 in 1909; 47.3/10,000 in 1927; and 42.03/10,000 in 1931-2. It is, however, unclear what ailments these prescriptions were issued to address.^(10)

The late 30s saw the isolation of the compounds responsible for vitamin D production. This, in turn, meant that cod liver oil's antirachitic properties could be isolated from the oil itself, and marked the end of cod liver oil's heyday as a medical treatment.

While cod liver oil's uses may have been exaggerated somewhat in home treatments, the fad thus occurred under the auspices of the international pharmaceutical and medical communities, and indeed seems to be largely supported by the conclusions of modern science.