Sinemet (carbidopa-levodopa) 25/100 immediate release (IR) tablets contain a yellow dye, D&C Yellow #10 Lake. Per the prescribing information, rash is a possible side effect (1).
There are a few case reports to support rash as a possible side effect in two publications. The first paper to cite this effect noted five patients who experienced rash associated with carbidopa/levodopa 25/100, and authors concluded the yellow dye in this formulation was responsible (2). Substitution with carbidopa/levodopa 25/250 or 10/100, formulations that do not contain the yellow dye, did not cause a rash.
The second paper reported two patients with rashes resulting from IR carbidopa/levodopa 25/100 (3). In both cases, stopping the drug resolved the rash. In one case, compounded pure levodopa and Stalevo 100 (carbidopa 25 mg/levodopa 100 mg/entacapone 200 mg), both of which are made without the yellow dye, were given in successive trials with no rash.
These are old reports, and searches for any more current information come up dry. The incidence is apparently very low, and the phenomenon not well known. I have seen this in patients and tend to try the CR, ER, or SA formulations, which do not contain the yellow dye, as alternatives.
Anecdotally, I have heard of allergic gastritis with IR carbidopa/levodopa, but have not seen this reported in literature. Some patients may also experience nausea with the IR, but not other formulations of carbidopa/levodopa.
1. https://www.merck.com/product/usa/pi_circulars/s/sinemet/sinemet_pi.pdf
2. Goetz CG. Skin rash associated with Sinemet 25/100. N Engl J Med 1983;309:1387-88.
3. Chou and Stacey. Skin rash associated with Sinemet does not equal levodopa allergy. Neurology. 2007 Mar 27;68(13):1078-9.
