When can I start taking multivitamins and cod liver oil after PRP session for hair loss?

I generally recommend that patients stop taking anything that might increase the risk of bleeding or bruising at least one week ahead of any treatment that might cause bruising like PRP. 2 weeks is even better. Supplements are unlikely to change the effectiveness of PRP, but could increase the bleeding/bruising risk. 
I usually recommend a series of 3 PRP treatments spaced one month apart followed by maintenance treatments every 6 months.
All the best,

In my opinion, there should be no interaction between PRP and cod liver oil or multivitamins. The protocol of using PRP for hair restoration has not been standardized. Is my feeling that at least two, probably three or four, injections are PRP are needed on a monthly basis to start the process. Maintenance treatments can be performed anywhere between 3 and 9 months apart. Each patient is individual and therefore the doctor and patient need to discuss expectations and develop  a treatment program.PRPHAIRMD.COM 

There is no standard protocol at this time for PRP treatments and certain medications however since PRP involves inflammatory responses as well as involves multiple injections it usually is recommend to stop taking NSAIDS and any other blood thinning medications (fish oil, cod liver, multivitamins-since ingredients vary and may contain blood thinning medications) are usually discontinued about a week before. Mainly this is done to prevent excessive bruising and bleeding as well as to reduce any dampening effect to the antiinflammatory response you are wanting to achieve with PRP. Recommendations to resume medications is about a week after the treatment. Part of that reasoning is that you do not want to have possible anything interfered with PRP process.
Amount of sessions will also vary by how severe your hair loss is as well as if you are a man or women. Women typically require less treatment. For moderate hair loss, 2-3 treatments sessions 1-2 months apart can be enough to jumpstart hair growth from viable hair follicles. It can take up to 3 months to notice the biggest difference. Maintenance treatments yearly are recommended to maintain results once desired results are achieved.