The Africa-focused oil group's share price was 7.43 per cent down, at 149.5p in mid-morning trading.
Tullow warned that "banking discussions with regard to March 2016 re-determination have begun," as it reported that it finished 2015 with net debt of $4bn, with "significant facility headroom and free cash of $1.9bn".
The company also reported a loss after tax of $1.04bn (£720m) in 2015, 37 per cent less than the $1.6bn it lost in 2014.
Meanwhile the company said operating losses nearly halved to $1.09bn. In 2014 the company reported an operating loss of $1.96bn.
"Today's results demonstrate that Tullow adjusted well to low oil prices in 2015. We secured current and future cash flow through good operational delivery in West Africa, continued to build our resource base in East Africa, significantly cut costs across the group and benefited from our strong hedging position," said chief executive Andy Heavey.
"Our challenge in 2016 is to be equally robust in responding to the uncertainties that remain in the sector."
Tullow was given some good news last year when it agreed terms with lenders to keep its borrowing limit at $3.7bn, but that hasn't helped shares in the meantime, as the price of oil continued to slide.
The falling price of oil has also had an impact on its write-offs. The company said it had written off exploration costs of $749m.
With that came a cut in capital expenditure: in 2015, capex came to $1.7bn, with $1.1bn forecast for 2016 - but it added there is "work ongoing to potentially reduce [capex] to $900m".
It said it could potentially reduce annual capital expenditure to $300m from 2017 if the low price of oil continues.
"Tullow’s results underline the issues surrounding the sector, with the sharp decline in the oil price impacting financials against a backdrop of high operating costs and gearing. Despite a challenging 12 months, production remains within guidance," said Cantor Fitzgerald's Sam Wahab.
"Nevertheless, we still have concerns regarding the company’s leverage position, which we believe is a key factor in the company announcing that it will further curtail capex in 2016," he added