Snake-bitten conservationist Pete Bethune has taken a turn for the worse yesterday and doctors increased his drug dose as a the "ton" of poison still in his body caused his blood pressure to hike.

A drowsy Bethune, slurring his words, provided his daily video health update from a Costa Rica hospital bed earlier today.

The author, TV host and ship captain, originally from Hamilton, was bitten by a Fer-de-lance viper in the Costa Rican jungle on Boxing Day.

"Last night was a **** night actually, I had a lot more pain and the doctors have come back and said there's still a **** ton of poison - well they didn't use those words - but there's still a lot of poison inside me," Bethune said.

"There's still a lot of internal bleeding.

"So today they've sort of upped the drug rates again and I'm more drowsy than usual."

At one point in the video message Bethune loses his train of thought and has to stop, saying "I can't even think".

The Fer-de-lance viper is highly venomous and is responsible for the most snake bites of any species in Costa Rica.

Conservationist Pete Bethune's left leg has doubled in size since being bitten by a poisonous snake in Costa Rica. Photo / Supplied

Bethune said doctors were "back monitoring me every hour" and his blood pressure had risen.

"Blood pressure's gone back up, was at about 100/60, which is sort of typical for me. It's back up to about 130, 135 now. My pulse is up to about 140. Normally, it's a lot less than that.

"So I'm not out of the woods yet, I'm afraid. I hoped I could message you with lots of good news but it's not good news at all."

In some better news for the 55-year-old, he went to the toilet for the first time yesterday since the accident.

"On a positive note I did manage to go to the toilet. It was more relief than anything, it had been nagging at me for a couple of days the fact that I haven't been able to go to the toilet," Bethune said.

He had also experienced some anatomical improvements.

"Oh my balls aren't as black as they were either," Bethune said.

"They've given me a bag of ice for my balls. Apparently for snake bite victims where you're putting a large volume of fluid through for them to go a bit black with the catheter system."

He said despite not being "out of the woods", his doctors "still seem confident".