Common side effects

Most common side effects are predictable consequences of opioid pharmacological actions and include nausea, vomiting, constipation, pruritus, dizziness, dry mouth and sedation.

Side effects are extremely common with opioid therapy.
Between 50% and 80% of patients in clinical trials experience at least one side effect from opioid therapy, however in everyday use the incidence may be even higher.

Adverse events frequently lead to discontinuation of opioid therapy. Most side effects, with the exception of constipation and itching, improve shortly after initiation of treatment or following an intended dose increase.

Constipation and itching tend to persist throughout treatment and may require long-term management.

Opioids have multiple effects on respiratory physiology, including decreased central respiratory drive, respiratory rate, and tidal volume. They also increase airway resistance and decrease the patency of the upper airways. The consequence of all of these effects may lead to ineffective ventilation and upper airway obstruction in susceptible individuals.
Respiratory depression is a much-feared harm associated with the use of opioids. It is mostly a concern in acute pain management where patients have not developed tolerance. For persistent pain it is most likely to be a potential problem if there has been a large, often unintended dose increase, or changes in formulation or route of administration.
Opioids can cause irregular respiratory pauses and gasping may lead to erratic breathing and significant variability in respiratory rate. The respiratory effects of opioids are more pronounced during sleep. Fatalities have been reported in patients with obstructive sleep apnoea who are prescribed opioids and sleep apnoea may be a relative contraindication to opioid therapy. This is particularly important if patients are taking other central respiratory depressants such as benzodiazepines. If opioids are prescribed to patients with obstructive sleep apnoea they will need up to date assessment of nocturnal respiratory function and should be compliant with therapy for this eg, continuous positive airway pressure. Patients with sleep apnoea being prescribed opioids will need regular and detailed assessment of treatment.
There is little evidence that, in equi-analgesic doses, commonly used opioids differ markedly in the incidence of their side effects.
Patients using intermittent opioid dosing regimens might not become tolerant to side effects.
Increased absorption may occur from transdermal opioid formulations with a fever or other intercurrent illness, and if the patient is exposed to external heat, for example a hot bath or sauna. If concerns arise, closer patient monitoring will be required.
Inadequate management of side effects (intractable constipation, faecal impaction, bowel obstruction) and consequences of opioid treatment (falls, fractures and acute confusional state) may contribute to unplanned hospital admissions and contribute to the overall costs associated with opioid treatment

Reference: Opioids Aware: A resource for patients and healthcare professionals to support prescribing of opioid medicines for pain.