The emergency calls arrive at all hours, especially in the summer. Pablo Roig weaves his ambulance through heavy traffic and past crowded beaches. He arrives at an increasingly familiar scene on Ibiza, Spain’s famed party island: drug-related distress at a nightclub.

“There are days when we’re so busy you can barely even stop to eat or have a coffee,” Mr. Roig, a 47-year-old ambulance technician, said.

Emergency calls involving partygoers at Ibiza nightclubs have become so frequent that the island’s public ambulance service is at risk of collapse, the local health technicians union said. During peak season, more than a quarter of all calls for ambulances are to nightclubs, and they often involve foreign visitors, straining resources for the island’s 160,000 full-time residents, the union said.

“Sometimes we go to the same nightclub three or four times in one night,” said José Manuel Maroto, a representative for the union. “There are nightclubs where we have to go to pick up an intoxicated patient every day.”

The ambulance crisis in Ibiza, one of the engines of Spain’s tourism industry with around 3.3 million visitors last year, is the latest example of tensions in Europe over foreign travelers as summer crowds peak. Anti-tourism protests have erupted recently in Spain, Italy and Portugal, with demonstrators complaining that overtourism is stretching public resources and driving up the cost of living.

Ibiza, a Mediterranean island off Valencia, has been a magnet for clubbers since at least the 1970s, when its first nightclubs were built on a hippie culture that thrived in the shadow of the Francisco Franco dictatorship.

The vibrant nightlife and good-vibes image have long come with drug use. But Mr. Matoro, 52, who was born on the island and has worked in ambulances for 32 years, said that soaring prices for admission to Ibiza’s so-called “superclubs” and the growing availability of cheap experimental drugs have contributed to a crisis.

General tickets to the superclubs, which can hold up to 10,000 people, go for upward of 100 euros ($116), and drinks there can cost 25 euros apiece. Drugs, Mr. Matoro said, are a more affordable alternative to alcohol on what can be an expensive night out.

Drugs are illegal on the island but all manner of them are used, Mr. Matoro said, including Ecstasy, cocaine, tusi (also known as pink cocaine), amphetamines and psychedelics — and they fall in and out of fashion. “Right now, ketamine is popular,” Mr. Matoro said.

Each year before the summer party season, he said, the island’s health workers try to predict which drugs will be most in use, so they can be prepared with the right medications.

“It’s a bit of a game of cat and mouse,” Mr. Matoro said. “They’re ahead of the curve, and we’re lagging behind, trying to figure out how to provide health care solutions for these types of patients.”

Eight ambulances and mobile intensive-care units typically work every night, and field an average of about 70 emergency calls, Mr. Matoro said.

Ambulance workers say emergency calls from nightclubs are particularly challenging because they often involve an unconscious person, making them a “priority alert” because of the risk that the patient could go into cardiac arrest or die.

But when emergency workers respond at clubs and try to figure out if a person was sickened by drugs, their questions are not always welcomed by partygoers who fear getting caught breaking the law, said Mr. Roig, the ambulance technician.

“Sometimes we’re met with aggression, both physical and verbal,” he said.

A typical drug-related call takes an hour to an hour and a half to resolve, emergency workers said, as the patient is treated and stabilized before being transferred to one of the island’s two hospitals. During the summer, residents of Ibiza regularly complain to ambulance employees about long waits, workers said.

The state broadcaster, Televisión Española, recently aired a story on the issue in which several residents grumbled about the wait times. “It feels pretty bad — sometimes ambulances aren’t available,” one said.

The government health service responsible for the Balearic Islands, which include Ibiza, did not respond to a request for comment.

Virtually all health care in Spain is free for residents, and for years foreign patients have often managed to be treated without paying.

Ibiza’s nightclubs are legally required to have medical workers on staff, including nurses. Now the health services union is demanding that nightclubs be required to contract private ambulances to handle distress calls in order to relieve some of the strain on public services.

“It’s unfair that nightclubs that earn millions of dollars per year are dumping this problem on the public health system,” Mr. Maroto said. “In the end, the bill is paid by residents on the island.”

Amnesia Ibiza, one of the oldest clubs on the island with a capacity of 5,000, said in a statement that it employed health professionals “who are prepared to handle any incidents that may arise within the club.” Last year, it said, the club had to call a public ambulance “on only 19 occasions.”

Pacha, which can hold 3,000 people, said in a statement that “just two” medical cases had required public ambulance services so far this summer, and that it was “committed to easing the pressure on public services.”

For Mr. Roig, the difference between drug-related calls and other emergencies is simple. “One is completely preventable,” he said.