The most significant side effect was that three patients complained of elation during the first days of treatment. In rodents, SAMe bounces back in the brain after L-dopa, but doesn't in the liver. This theory has never been proven or disproven. The researchers attempted to prove their theory by injecting huge amounts of SAMe directly into the brains of rodents so as to produce PD-like symptoms. One problem with the study is that such huge amounts of any substance injected directly into an organ can cause severe damage.
Many different substances can create PD symptoms by depleting dopamine in the brain.
Manganese chloride is one of those substances. Iron is another. Another is MPTP, which is used by researchers in studies to create PD in animal models. The one published study on giving SAMe to PD patients does not support the theory that SAMe is detrimental to PD patients. On the contrary, the results of that study show a beneficial effect.