Can folic acid be substituted with folinic acid?

• Folic Acid and folinic acid are forms of vitamin B9. 
• The human body needs folate to perform many functions, including cell division, growth, and the production of new red blood cells. 
• Folinic acid is chemically different to folic acid but both work in a similar way
• Folinic acid is a metabolically active reduced form of folate that bypasses dihydrofolate reductase
• It has an important role in the treatment of methotrexate overdose and acute bone marrow toxicity, due to its faster action and independence of dihydrofolate reductase

Substitution in Toxoplasmosis:

• Toxoplasma cannot take in folinic acid, but can absorb folic acid, thereby partially bypassing the synergistic activity of pyrimethamine and sulphonamide.
• Hence folinic acid should be used to prevent toxicity and folic acid should be avoided since it might thwart the therapeutic effect.

References:

1. Toxoplasmosis A Comprehensive Clinical Guide by D.H.M Joynson and T.G Wreghitt 2005 pg 322