PCO2 is the partial pressure of carbon dioxide in the blood plasma.The pH of the extracellular fluids can thus be controlled by the regulation of 
  
    
      
        
          P
          
            
              
                
                  C
                  O
                
              
              
                2
              
            
          
        
      
    
    {\displaystyle P_{{\mathrm {CO} }_{2}}}
   and the other metabolic acids.
The FDA lists the following:For subjects with chronic idiopathic constipation taking Amitiza:

Nausea ~ 29% (4% were severe, and 9% of patients discontinued treatment due to nausea. The rate of nausea was lower among male (8%) and elderly (19%) patients. No patients in the clinical studies were hospitalized due to nausea.) Diarrhea: ~12%  (2% were severe, and 2% of patients discontinued treatment due to diarrhea)
Several less common adverse reactions (<1%).For opioid-induced constipation:

Nausea: ~ 11%; 1% severe nausea and 2% discontinued treatment due to nausea. Diarrhea: ~ 8%; 2% severe diarrhea and 1% of patients discontinued treatment due to diarrhea. Less common adverse reactions (<1%): fecal incontinence, blood potassium decreased.For subjects with irritable bowel syndrome with constipation:

Nausea: ~ 8%; 1% severe nausea and 1% discontinued treatment due to nausea. Diarrhea: ~ 7%; <1% of patients had severe diarrhea and <1% of patients discontinued treatment due to diarrhea. Less common adverse reactions: <1%A 2018 pooled analysis from three phase III, randomized, double-blind, placebo-controlled studies on usage for Opioid-Induced Constipation, found that the numbers of patients reporting adverse effects were similar in both the lubiprostone and placebo treatment groups for all opioid classes (P ≥ 0.125); however, gastrointestinal adverse effects were reported more frequently by those receiving lubiprostone than 2 of the 3 opioid groups. The most commonly reported TEAEs in the lubiprostone treatment groups were nausea (13.4%–18.1%), diarrhea (1.2%–13.9%), and abdominal pain (4.7%–5.6%).