(wanted to make this a separate topic. most of the articles have appeared in foreign journals. Our neurologist has never mentioned the need to supplement with B12 and folic acid. Has anyone on this forum been advised to use these with Sinemet? thanks, madelyn)

folic acid and B 12
there are a number of studies noting use of supplements Vitamin B 12 and folic acid in instances of B 12 deficiencies to decrease homocysteine levels that result from use of Levodopa. Levodopa reportedly interferes with folate metabolism and B 12 function.

Parkinsonism Relat Disord. 2008;14(4):321-5. Epub 2007 Dec 4.
Folate and vitamin B12 levels in levodopa-treated Parkinson's disease patients: their relationship to clinical manifestations, mood and cognition.

Abstract
We tested the hypothesis that mood, clinical manifestations and cognitive impairment of levodopa-treated Parkinson's disease (PD) patients are associated with vitamin B12 and folate deficiency...Levodopa-treated PD patients showed significantly lower serum levels of folate and vitamin B12 than neurological controls, while depressed patients had significantly lower serum folate levels as compared to non-depressed. Cognitively impaired PD patients exhibited significantly lower serum vitamin B12 levels as compared to cognitively non-impaired. In conclusion, lower folate levels were associated with depression, while lower vitamin B12 levels were associated with cognitive impairment. The effects of vitamin supplementation merit further attention and investigation.
PMID: 18055246 [PubMed - indexed for MEDLINE]


The influence of levodopa and the COMT inhibitor on serum vitamin B12 and folate levels in Parkinson's disease patients.
...Our findings show that levodopa-treated Parkinson's disease patients have low folate (p < 0.0007) and vitamin B12 levels (p < 0.0003). They also demonstrate that the addition of a COMT-i to levodopa + DDC-i treatment causes lower serum vitamin B12 (p < 0.03) and folate levels (p < 0.005) than levodopa + DDC-i treatment alone. We suggest supplementary treatment with vitamin B12 and folic acid in these situations.
http://www.ncbi.nlm.nih.gov/pubmed/17565222

Neurol Neurosurg Psychiatry 2003;74:549 doi:10.1136/jnnp.74.4.549
Correspondence
Benefit of folic acid supplementation in parkinsonian patients treated with levodopa
T Müller, D Woitalla, W Kuhn
+ Author Affiliations

Department of Neurology, St Josef Hospital, Ruhr University Bochum, Gudrunstrasse 56, 44791 Bochum, Germany
Correspondence to:
 Dr T Müller; 
 thomas.mueller@ruhr-uni-bochum.de
We read with interest the recent excellent review by Reynolds on the role of folic acid and the risks and benefits of its supplementation in the nervous system.1 It emphasises the beneficial importance of folate on the numerous methylation processes in combination with S-adenosylmethionine (SAM), which donates its methyl group to prevent hyperhomocysteinaemia.1 However SAM deficiency, which is associated with, for example, cognitive decline and/or mood disturbances, and increased total homocysteine levels, which support onset of vascular disease, may also caused by drugs, for example, levodopa.2,3 Levodopa is administered with dopa decarboxylase inhibitors (DDI) to prevent its peripheral degradation. This increases conversion of levodopa to 3-O-methyldopa (3-OMD) by the ubiquitious enzyme catechol-O-methyltransferase (COMT) in blood, peripheral tissues and in nigrostriatal neurons.2,3 COMT requires Mg2+ as cofactor and SAM as methyl donor. Thus O-methylation of levodopa to 3-OMD is associated with conversion of SAM to S-adenosylhomocysteine and subsequently homocysteine.

http://jnnp.bmj.com/content/74/4/549.1.full

(B12 supplement should be "under the tongue" or sublingual form. stomach acids render an oral form useless.
For those individuals who have had DNA profiling, check for polymorphisms of the methylenetetrahydrofolate reductase (MTHFR C677T), methyltetrahydrofolate-homocysteine methyltransferase (MTR A2756G), and 5-methyltetrahydrofolate-homocysteine methyltransferase reductase both associated with folate metabolism and in addition, check (MTRR A1049G and C1783T) associated with processing of B12 in the bodY. If you have mutations in the MTHFR gene, must use a breakdown product of folic acid, methyl folate, and folinic acid if one has the other mutation. Mutations in MTRR requires a B 12 supplement.