Several recent studies confirm past research on fumaric acid’s ability to support skin health. In a study published by the British Journal of Dermatology, the researchers analyzed the records of patients who had received fumaric acid for severe psoriasis, either alone or combined with other systemic medications.

Of 12 patients treated with fumaric acid, 11 patients experienced an improvement in psoriasis after starting fumaric acid. Nine patients received fumaric acid in combination with other systemic therapies including cyclosporin, acitretin, hydroxyurea and methotrexate. Seven of the patients were able to reduce the dosage of the pharmaceuticals while taking fumaric acid. In two patients, severe psoriasis was controlled using fumaric acid alone.

The researchers concluded, “Fumaric acid appears effective and less toxic than other systemic treatments for psoriasis. Fumaric acid esters were used successfully in combination with other systemic agents and generally enabled the doses of the more hazardous drugs to be reduced.”

Studies have also been made for the effectiveness of fumaric acid esters, in particular dimethylfumarate (DMF), in the treatment of moderate to severe plaque psoriasis intolerant and/or resistant to other conventional systemic therapies. The findings suggest that [the fumaric acid ester] dimethylfumarate is a safe, effective and well-tolerated long-term oral treatment worthy of consideration for selective patients.

Side effects of the use of Fumaric acid, mostly mild, consisted of flushing (55 percent), diarrhea (42 percent), nausea (14 percent), tiredness (14 percent) and stomach complaints (12 percent). Lymphocytopenia (a deficiency of lymphocytes in the blood) has also been observed in quite a few patients during therapy with fumaric acid esters.

• Maleic acid is a less stable molecule than fumaric acid. The difference in heat of combustion is 22.7 kJ·mol⁻¹.
• Maleic acid is more soluble in water, than fumaric acid.
• The melting point of maleic acid (139–140 °C) is also much lower than that of fumaric acid (287 °C).
• Intramolecular hydrogen bonding that takes place in maleic acid at the expense of intermolecular interactions is not possible in fumaric acid for geometric reasons.

The major industrial use of maleic acid is its conversion to fumaric acid by isomerization. This isomerization is catalyzed by a variety of reagents, such as mineral acids and thiourea.

Maleic acid and fumaric acid do not spontaneously interconvert because rotation around a carbon carbon double bond is not energetically favourable. However, conversion of the cis isomer into the trans isomer is possible by photolysis in the presence of a small amount of bromine. Light converts elemental bromine into a bromine radical, which attacks the alkene in a radical addition reaction to a bromo-alkane radical; and now single bond rotation is possible. The bromine radicals recombine and fumaric acid is formed.

There is another way in which maleic acid can be coverted into fumaric acid. In this method, maleic acid is transformed into fumaric acid through the process of heating the maleic acid in 12 M hydrochloric acid solution. Reversible addition (of H⁺) leads to free rotation about the central C-C bond and formation of the more stable and less soluble fumaric acid.