Pregnancy and breastfeeding
The safety of using omeprazole has not been established in pregnant or breastfeeding women. Epidemiological data do not show an increased risk of major birth defects after maternal use of omeprazole during pregnancy.No clinical trials have deeply evaluated the potential consequences of the use of omeprazole in breastfeeding. However, the pharmacokinetics of the omeprazole molecule strongly suggest the safety of omeprazole use during breastfeeding:

Omeprazole has a high plasma protein binding rate (95%), indicating that a little amount of drug is transferred to the milk duct during breast milk formation. Omeprazole needs to be administered in an enteric-coated formulation due to its rapid degradation in the acidic conditions of the stomach. This suggests that most of the free molecules ingested by the infant are likely degraded before being absorbed.Omeprazole at normal doses is likely safe during breastfeeding. Interactions
Important drug interactions are rare. However, the most significant major drug interaction concern is the decreased activation of clopidogrel when taken together with omeprazole. Although still controversial, this may increase the risk of stroke or heart attack in people taking clopidogrel to prevent these events. This interaction is possible because omeprazole is an inhibitor of the enzymes CYP2C19 and CYP3A4. Clopidogrel is an inactive prodrug that partially depends on CYP2C19 for conversion to its active form.
Adverse effects
Adverse effects occurring in at least 1% of people include:
Central nervous system: headache (7%), dizziness (2%)
Respiratory: upper respiratory tract infection (2%), cough (1%)
Gastrointestinal: abdominal pain (5%), diarrhea (4%), nausea (4%), vomiting (3%), flatulence (3%), acid regurgitation (2%), constipation (2%)
Neuromuscular and skeletal: back pain (1%), weakness (1%)
Dermatologic: rash (2%)Other concerns related to adverse effects are:

Recurrence of Clostridium difficile associated diarrhea
Osteoporosis-related fractures
HypomagnesemiaConcern has been expressed regarding vitamin B12 and iron malabsorption, but effects seem to be insignificant, especially when supplement therapy is provided.Since their introduction, proton-pump inhibitors (PPIs, especially omeprazole) have also been associated with several cases of acute interstitial nephritis, an inflammation of the kidneys that often occurs as an adverse drug reaction. Long-term use
Long-term use of PPIs is strongly associated with the development of benign polyps from fundic glands (which is distinct from fundic gland polyposis); these polyps do not cause cancer and resolve when PPIs are discontinued. No association is seen between PPI use and cancer, but use of PPIs may mask gastric cancers or other serious gastric problems and physicians should be aware of this effect.There is a possible association between long term use and dementia which requires further study to confirm.A review article in U.S. Pharmacist in 2013 states that long-term use of PPIs is associated with decreased calcium absorption (causing increased risk of osteoporosis and fractures), decreased magnesium absorption (causing electrolyte disturbances), and increased risk of certain infections such as C. difficile and community-acquired pneumonia. They hypothesize that this is due to decreased stomach acid production.