Milk has a lower pH and protein binding capacity than plasma, so drugs distribute into milk depending on their lipophilicity and plasma protein binding. Highly plasma protein-bound drugs remain in the bloodstream and have lower concentrations in milk. All antithyroid drugs are secreted in milk, so breastfeeding is not recommended for women taking these medications. The extent a drug transfers to milk is affected by its pKa, protein binding, and lipophilicity. Drugs that are basic, have low plasma protein binding, and are lipophilic will have higher milk concentrations. The timing of breastfeeding should be considered when taking certain drugs.