Try a alternative approach, red light/laser light for the abdomen has worked and info on you tube. Also massaging the tummy along with exercise. Diet high fibre will also help.
I’ve had bad constipation for years (9 yrs with pd) and they put me on one called Laxido. That sorted me out virtually overnight and is brilliant. Makes it soft but no sudden movements which always concerned me. Hope that helps
Iron tablets aren’t great with levadopa. Good idea to check this.
I find laxido best, together with fruit daily and golden linseeds on cereal.
I take fibre and drink a lot of water. Stay active as much as possible and try and be vertical more often. Ive tried senna but that seemed less effective for me. the one thing I resort to is using a saline personal enema. This works if evrything else does not. The type I use is made by unison and comprises a sterile 20 cc saline solution in a little plastic globe with a twist off top spout. You use a little vaseline and aim the spout twords your belly button from below at the anus. this improves with practice. Squeezing the saline inside. In 30 seconds to a minute you poo. Good luck.
I’ve recently been put on Laxodi which has proved better for me as I no longer cough, whereas with Isphaghula Husk I was coughing quite a lot. The only comment I can make about Laxodi is that the orange flavour doesn’t taste of orange !!
Les
My husband has taken Laxido for years, prescribed by GP, with no problems.
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Hubby drinks a daily glass of carrot juice eats a fiber rich diet, hydrates well & takes magnesium L- threonate.
Now he’s regular like a swiss watch 
I have taken 15ml Lactulose twice a day for just under 2 years, from long before being diagnosed with PD and I rarely get a day when I dont “go”. Its on prescription for me but if you have to pay its about £10 a bottle and I go through two bottles a month. If I am having a difficult time with constipation I up the dose a little. My doc says its safe to take forever because all it does is absorb water from your intestines. I also have All Bran, Prunes and a Kiwi Fruit for breakfast every day without fail.