Smith suffered his third concussion since February - and sixth in the past five years - during the All Blacks' 30-15 win against the British and Irish Lions in Auckland on Saturday night.
Fulcher, an Auckland-based sport and exercise physician for Axis Sports Medicine Specialists, said the frequency of Smith's injuries were "concerning", and he would discuss retirement or a break from the game if he were to sit down with the 31-year-old.
"If you're having more and more concussions over a period of time, you might decide - on the balance of probability - you want to be careful and protect your long-term well-being and you might not go back to sport," Fulcher said.
"It's a very personal thing. You see how you go, you treat the problem as best you can and there is always that degree of risk with re-injury. It's just whether you accept that risk or not."
Smith's latest head knock was just three weeks after his last - playing for the Highlanders against the Crusaders in Christchurch - while he missed two games after suffering concussion against the Chiefs in the opening round of Super Rugby.
When he returned from the blow against the Chiefs, Smith told Stuff he suffered from a "wee bit" of memory loss.
"I just felt really tired. So I had to make sure I was resting. It just needed a bit of time to clear and I'm just really lucky that it did clear up," Smith, who also missed two weeks with concussion last year, said in March.
All Blacks coach Steve Hansen has expressed his concern for Smith, who has been sent home and is under concussion protocol. There is still no word on how long he is expected to be sidelined for.
Fulcher, who helped Accident Compensation Corporation develop their guidelines around sport concussion, said a quantum leap had been made in terms of managing concussion the past 10 years, but there were still some unknowns.
"We don't know whether there is an absolute number of concussions that people must retire after," he said.
"But if you're having concussions that are happening more frequently and with less impact and less force, then potentially that might mean there is an on-going susceptibility to having more injuries.
"I know nothing about his [Smith's] individual case, but in general terms, people often will talk about getting concussions with less force and they feel like they are getting them easier. That potentially means either the original injury wasn't managed conservatively enough, or maybe the susceptibility is increasing for further injury."
As concerning as six concussions in five years sounds, Fulcher warned against looking at the numbers and stating Smith should never play again.
He said each bout of concussion was treated as a new injury and concussion sufferers were not treated with a formulaic approach.
"It is potentially concerning, but each individual injury gets managed on its own merits, essentially," Fulcher said.
"There are people that have had one concussion and probably should never play again."
Smith's concussion problems bring back memories of another All Black fullback - Leon MacDonald - whose career was littered with concussions.
The now Crusaders assistant coach retired in 2010, a decision prompted by a serious concussion he suffered while playing for Japanese club Kintetsu the previous year.
Fulcher would be "stunned" if Smith played in the second test against the Lions in Wellington on Saturday night, and expects the All Blacks' medical team to take a conservative approach with him.
"They won't be pushing him . . . they will be measuring his brain function and his memory and all those sorts of parameters," he said.
"And if that's all going well, then they will make an informed decision . . . let's get back into it or not.
"In situations like this, it is concerning. But equally I have good faith that the right decisions are being made.
Photo by: HANNAH PETERS/GETTY IMAGES (All Blacks vice-captain Ben Smith leaves Eden Park for a concussion test during the first half of Saturday's test match against the British and Irish Lions at Eden Park).