Regimens are defined using the `new_regimen()`

function.
You should provide information on the dose amount, how many doses, and
the dosing interval. However, how you provide that information is very
flexible. You can e.g. specify:

```
<- new_regimen(
reg1 amt = 100,
interval = 12,
n = 5
)
```

which will administer 5 doses of 100 mg (or whatever the unit is), at an interval of 12 hours. The same can however also be specified as:

```
<- new_regimen(
reg1 amt = 100,
times = c(0, 12, 24, 36, 48)
)
```

or:

```
<- new_regimen(
reg1 amt = c(100, 100, 100, 100, 100),
times = c(0, 12, 24, 36, 48)
)
```

Be careful: if the length of the `amt`

vector is not the
same as the lenght in `times`

, the function will repeat the
first `amt`

specified. The same goes for the arguments
`type`

, `cmt`

, and `t_inf`

.

By default, the `new_regimen()`

function assumes the dose
is given as bolus. If you want to specify that these should be given as
infusion, specify the `type`

and `t_inf`

arguments:

```
<- new_regimen(
reg1 amt = 100,
times = c(0, 12, 24, 36, 48),
type = "infusion",
t_inf = c(1, 1, 1.5, 1.5, 1)
)
```

When you created the model using `new_ode_model`

, you also
specified a default dosing compartment. If you didn’t specify the
`dose`

argument, the default dosing compartment is 1. Now,
should you want to administer the dose into a different compartment, you
can override it with the `cmt`

argument:

```
<- new_regimen(
reg1 amt = 100,
times = c(0, 12, 24, 36, 48),
cmt = c(1, 1, 2, 2, 3)
)
```

Which will administer the doses to compartments 1, 1, 2, 2, and 3, respectively (of course this is a hypothetical example).

*Note: PKPDsim doesn’t care about what unit the dose
is in, you have to define this yourself in conjunction with the model
code and observation scale.*

Instead of simulating the full concentration time course for a give
model, if you want to simulate the PK(PD) at steady state, you can also
specify the `ss=TRUE`

argument to `new_regimen()`

,
which will then simulate the regimen at steady state. By default, steady
state is assumed to occur after 5 days (PKPDsim doesn’t check if steady
state is reached!), but if you want to decrease or increase this you can
do so by specifying the `n_ss`

argument which takes the
number of doses to simulate before assuming steady state.

```
<- new_regimen(
reg_ss amt = 100,
interval = 12,
n = 5,
ss = TRUE,
n_ss = 10
) reg_ss
```

`## Note: Steady state regimen, steady state assumed to be reached at t = 0.`

```
## t type amt t_inf
## 1 0 bolus 100 0
## 2 12 bolus 100 0
## 3 24 bolus 100 0
## 4 36 bolus 100 0
## 5 48 bolus 100 0
```